Fix zero value omission for BackendService CDN Policy TTLs

[burczyk]

Summary of changes:

1. Custom Expander Template: Created backend_service_cdn_policy_ttl.go.tmpl using d.GetOkExists to distinguish between explicit 0 and omission. It returns a pointer *int which allows Terraform to distinguish 0 from nil (omitted).
2. YAML Updates: Registered this custom expander in BackendService.yaml and RegionBackendService.yaml for defaultTtl, maxTtl, and clientTtl.
3. Tests Added: Added backend_service_cache_zero_ttl and backend_service_cache_omitted_ttl samples to verify the fix and prevent regressions.
4. Verification: All related tests passed, including the one that was failing with Option 2 (TestAccComputeBackendService_backendServiceCacheIncludeHttpHeadersExample), confirming that omitted TTLs are no longer implicitly sent as 0.

Linked Issues:

Fixes https://issuetracker.google.com/issues/498433517

Release Note Template for Downstream PRs (will be copied)

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compute: fixed omission of explicit `0` values for `default_ttl`, `max_ttl`, and `client_ttl` in `cdn_policy` for `google_compute_backend_service` and `google_compute_region_backend_service`