Add data source for Serverless VPC Access Connector.

[lescoggi]

Fixes hashicorp/terraform-provider-google#8827

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_vpc_access_connector`

[modular-magician]

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

I've detected that you're a community contributor. @rileykarson, a repository maintainer, has been assigned to assist you and help review your changes.

❓ First time contributing? Click here for more details

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Your assigned reviewer will help review your code by:

• Ensuring it's backwards compatible, covers common error cases, etc.
• Summarizing the change into a user-facing changelog note.
• Passes tests, either our "VCR" suite, a set of presubmit tests, or with manual test runs.

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[modular-magician]

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

Diff report:

Terraform GA: Diff ( 4 files changed, 145 insertions(+))
Terraform Beta: Diff ( 4 files changed, 145 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2171
Passed tests 1931
Skipped tests: 236
Failed tests: 4

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccPrivatecaCertificateAuthority_privatecaCertificateAuthoritySubordinateExample|TestAccVPCAccessConnectorDatasource_basic|TestAccComputeInstance_soleTenantNodeAffinities|TestAccCGCSnippet_eventarcWorkflowsExample

[modular-magician]

Tests passed during RECORDING mode:
TestAccPrivatecaCertificateAuthority_privatecaCertificateAuthoritySubordinateExample[Debug log]
TestAccVPCAccessConnectorDatasource_basic[Debug log]
TestAccCGCSnippet_eventarcWorkflowsExample[Debug log]

Tests failed during RECORDING mode:
TestAccComputeInstance_soleTenantNodeAffinities[Error message] [Debug log]

Please fix these to complete your PR
View the build log or the debug log for each test

[rileykarson]

Generally LGTM, just one comment on the documentation!

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 4 files changed, 121 insertions(+))
Terraform Beta: Diff ( 4 files changed, 121 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2172
Passed tests 1932
Skipped tests: 238
Failed tests: 2

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccFirebaserulesRelease_BasicRelease|TestAccComputeInstance_soleTenantNodeAffinities

[modular-magician]

Tests passed during RECORDING mode:
TestAccFirebaserulesRelease_BasicRelease[Debug log]

Tests failed during RECORDING mode:
TestAccComputeInstance_soleTenantNodeAffinities[Error message] [Debug log]

Please fix these to complete your PR
View the build log or the debug log for each test

[rileykarson]

Caught one last thing, sorry for the extra round trip!

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 4 files changed, 130 insertions(+))
Terraform Beta: Diff ( 4 files changed, 130 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2174
Passed tests 1934
Skipped tests: 238
Failed tests: 2

Action taken

Triggering VCR tests in RECORDING mode for the tests that failed during VCR. Click here to see the failed tests

TestAccComputeInstance_soleTenantNodeAffinities|TestAccStorageTransferJob_transferOptions

[modular-magician]

Tests passed during RECORDING mode:
TestAccStorageTransferJob_transferOptions[Debug log]

Tests failed during RECORDING mode:
TestAccComputeInstance_soleTenantNodeAffinities[Error message] [Debug log]

Please fix these to complete your PR
View the build log or the debug log for each test