Add data source for retrieving a project iam custom role

[bestefreund]

Adds a new data source "google_project_iam_custom_role", allowing to retrieve a project iam custom role.

Fixes: hashicorp/terraform-provider-google#21777

Release Note Template for Downstream PRs (will be copied)

`google_project_iam_custom_role`

[github-actions]

Hello! I am a robot. Tests will require approval from a repository maintainer to run. Googlers: see go/terraform-auto-test-runs to set up automatic test runs.

@ScottSuarez, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

You can help make sure that review is quick by doing a self-review and by running impacted tests locally.

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 4 files changed, 148 insertions(+))
google-beta provider: Diff ( 4 files changed, 148 insertions(+))

[modular-magician]

Tests analytics

Total tests: 162
Passed tests: 130
Skipped tests: 30
Affected tests: 2

Click here to see the affected service packages

• resourcemanager

Action taken

Found 2 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

• TestAccDataSourceGoogleProjectIamCustomRole_basic
• TestAccEphemeralServiceAccountKey_basic

Get to know how VCR tests work

[modular-magician]

🟢 Tests passed during RECORDING mode:
TestAccDataSourceGoogleProjectIamCustomRole_basic [Debug log]

🟢 No issues found for passed tests after REPLAYING rerun.

---

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

🔴 Errors occurred during RECORDING mode. Please fix them to complete your PR.

View the build log or the debug log for each test