Add RuntimeConfig datasource

[upodroid]

Fixes: hashicorp/terraform-provider-google#7428

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).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• 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_runtimeconfig_config`

[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.

@c2thorn, please review this PR or find an appropriate assignee.

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 9 files changed, 128 insertions(+), 6 deletions(-))
Terraform Beta: Diff ( 10 files changed, 129 insertions(+), 7 deletions(-))

[c2thorn]

seems solid, a couple of small asks

[c2thorn]

/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=168092"

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 9 files changed, 127 insertions(+), 6 deletions(-))
Terraform Beta: Diff ( 9 files changed, 127 insertions(+), 6 deletions(-))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccRuntimeconfigConfigDatasource_basic|TestAccActiveDirectoryDomainTrust_activeDirectoryDomainTrustBasicExample|TestAccCloudRunService_cloudRunServiceMultipleEnvironmentVariablesExample|TestAccPrivatecaCertificateAuthority_privatecaCertificateAuthorityCmekExample You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=168096"