Add Terraform support for regional backend buckets

[modular-magician]

Description

This PR adds Terraform support for Regional Backend Buckets (google_compute_region_backend_bucket), enabling GCS integration with regional HTTP(S) load balancers.

What does this PR do?

• Adds new resource: google_compute_region_backend_bucket
• Supports both INTERNAL_MANAGED and EXTERNAL_MANAGED load balancing schemes
• Includes IAM policy support (beta version)
• Adds 3 documentation examples demonstrating different use cases

GCP API Reference

• API Endpoint: projects/{project}/regions/{region}/backendBuckets
• Documentation: https://cloud.google.com/compute/docs/reference/v1/regionBackendBuckets
• This is a Preview feature in GCP

Testing

End-to-end tested with GCP API:

• INTERNAL_MANAGED scheme: Successfully created in us-central1
• EXTERNAL_MANAGED scheme: Successfully created in us-east1
• Verified via gcloud beta compute backend-buckets describe
• All fields (bucketName, loadBalancingScheme, region) working correctly

Files Changed

• mmv1/products/compute/RegionBackendBucket.yaml - Resource definition
• mmv1/templates/terraform/examples/region_backend_bucket_basic.tf.tmpl - Basic example
• mmv1/templates/terraform/examples/region_backend_bucket_internal_lb.tf.tmpl - Internal ALB example
• mmv1/templates/terraform/examples/region_backend_bucket_external_lb.tf.tmpl - External ALB example

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`google_compute_region_backend_bucket`

Derived from GoogleCloudPlatform/magic-modules#16308