feat: add runtimeconfig variable data source

[brandonjbjelland]

What is this change?

This PR adds a data source for google_runtimeconfig_variable as requested on this issue.

Checklist

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes). Comment here.
• Generated Terraform, and ran rake test and rake lint to ensure it passes unit and linter tests. - unrelated failure in products/appengine/api.yaml.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

`google_compute_new_datasource`

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I have detected that you are a community contributor, so your PR will be assigned to someone with a commit-bit on this repo for initial review.

Thanks for your contribution! A human will be with you soon.

@ScottSuarez, please review this PR or find an appropriate assignee.

[brandonjbjelland]

@googlebot I signed it!

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 6 files changed, 123 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 6 files changed, 123 insertions(+), 1 deletion(-))

[brandonjbjelland]

Question: It looks like all the runtimeconfig resources are still beta in our docs but the related resources look like they landed in both the GA and beta providers (including the work here). Should the data source in this changeset only ship to beta, and if so, how to do that exactly?

[ScottSuarez]

/gcbrun

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=171669"

[ScottSuarez]

/gcbrun

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=171677"

[ScottSuarez]

/gcbrun

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=171678"

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=171681"

[ScottSuarez]

/gcbrun

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=171682"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 6 files changed, 49 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 7 files changed, 132 insertions(+), 2 deletions(-))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 6 files changed, 49 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 6 files changed, 131 insertions(+), 1 deletion(-))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 6 files changed, 49 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 6 files changed, 131 insertions(+), 1 deletion(-))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 6 files changed, 49 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 7 files changed, 132 insertions(+), 2 deletions(-))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 48 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 6 files changed, 131 insertions(+), 1 deletion(-))

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 48 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 7 files changed, 132 insertions(+), 2 deletions(-))

[ScottSuarez]

/gcbrun

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=171685"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 5 files changed, 48 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 6 files changed, 131 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccRuntimeconfigVariableDatasource_basic|TestAccEssentialContactsContact_essentialContactExample You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=171691"