feat(bigquery): Add example for creating a reservation assignment

[ccarpentiere]

Description

Fixes b/269302678

Note: If you are not associated with Google, open an issue for discussion before submitting a pull request.

Checklist

Readiness

• [X] Yes, merge this PR after it is approved
• [ ] No, don't merge this PR after it is approved

Style

• [X] My sample follows the rules described for Terraform in the Effective Samples style guide
• [X] My sample follows the other requirements and best practices in the Contributing guide

Testing

• [X ] I have performed tests described in the Contributing guide:

  • [X] Tests pass: terraform apply
  • [X] Lint pass: terraform fmt check

Intended location

• [X] Yes, this sample will be (or already is) included on cloud.google.com Location(s): https://cloud.google.com/bigquery/docs/reservations-get-started#assign_a_project_to_a_reservation

• [ ] No, this sample won't be included on cloud.google.com Reason:

API enablement

• [ ] If the sample needs an API enabled to pass testing, I have added the service to the Test setup file

Review

• [ ] If this sample adds a new directory, I have added codeowners to the CODEOWNERS file

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 1 region tag.

- [bigquery/bigquery_create_reservation_assignment/main.tf:17](https://github.com/ccarpentiere/terraform-docs-samples/blob/f134d5f98af420448adb27f9efd2264ae3d850bf/bigquery/bigquery_create_reservation_assignment/main.tf#L17), tag `bigquery_create_reservation_assignment`

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[ccarpentiere]

I can merge the new example into /bigquery_create_reservation/main.tf if you prefer. It will still need a test.yaml file I think. The sample refers to an example project to apply to the reservation assignment, which causes the terraform-docs-samples-int-trigger check to fail when it tries to create a reservation in that project.