Add locality_lb_policies field to google_compute_backend_service

[modular-magician]

This is a new optional field that allows configuring custom load balancer
policies in gRPC clients.

The field has been promoted to production, but at the moment requires a
GCP project to be allowlisted for it to be used. I have run the test I defined
for the field against an allowlisted test project.

part of #11519

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• [ X ] 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).
• [ X ] Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• [ X ] 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).
• [ X ] Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• [ X ] Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

compute: added `locality_lb_policies` field to `google_compute_backend_service`

Derived from GoogleCloudPlatform/magic-modules#5947

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