Add fields to support stateful cookie-based affinity

[modular-magician]

Add Terraform support for configuring stateful cookie-based affinity. It is added to both backend service and regional backend service. Connections bearing the same cookie will be served by the same backend VM while that VM remains healthy, as long as the cookie has not expired.
Fixes: hashicorp/terraform-provider-google#19647

• Searched through the issue tracker for an open issue that this either resolves or contributes.
• 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). - read only field
• 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)

compute: added enum STATEFUL_COOKIE_AFFINITY and `strong_session_affinity_cookie` field to `google_compute_backend_service` and `google_compute_region_backend_service` resource

Derived from GoogleCloudPlatform/magic-modules#11845

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