feat(spanner): support user_project_override in spanner iam resources

[timothyklee]

spanner_database_iam and spanner_instance_iam resources currently do not support user_project_override. This will inject the required X-Goog-User-Project header for that feature to work.

spanner: support user_project_override in `spanner_database_iam` and `spanner_instance_iam` resources

[github-actions]

Hello! I am a robot. Tests will require approval from a repository maintainer to run.

Googlers: For automatic test runs see go/terraform-auto-test-runs.

@BBBmau, 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, 53 insertions(+), 13 deletions(-))
google-beta provider: Diff ( 4 files changed, 53 insertions(+), 13 deletions(-))
terraform-google-conversion: Diff ( 2 files changed, 40 insertions(+), 8 deletions(-))

[modular-magician]

Tests analytics

Total tests: 48
Passed tests: 42
Skipped tests: 6
Affected tests: 0

Click here to see the affected service packages

• spanner

🟢 All tests passed!

View the build log

[github-actions]

@BBBmau This PR has been waiting for review for 3 weekdays. Please take a look! Use the label disable-review-reminders to disable these notifications.

[github-actions]

@GoogleCloudPlatform/terraform-team @BBBmau This PR has been waiting for review for 1 week. Please take a look! Use the label disable-review-reminders to disable these notifications.