feat(storage): Support GCS Rapid Cache Ultra CRUD Integration Tests - DRAFT

[xlai20]

Adds Integration Test Suite 3 (RCU CRUD Management APIs) for GCS Rapid Cache Ultra in integration-tests-storage.

Draft only; eventually will create PR to merge into google-cloud-rust

Covered Scenarios:

1. test_create_rapid_cache: Creates an RCU instance on an HNS regional bucket, polls LRO to completion, and verifies state running, cache type rapid-cache-ultra, and TTL 86400s.
2. test_create_rapid_cache_invalid_config: Verifies Code::InvalidArgument when creating with invalid duration or bad configuration.
3. test_create_rapid_cache_duplicate: Verifies Code::AlreadyExists on creating an RCU where one already exists.
4. test_get_rapid_cache: Retrieves RCU metadata and asserts all fields.
5. test_get_rapid_cache_non_existent: Verifies Code::NotFound on non-existent zone/cache.
6. test_list_rapid_caches: Streams Rapid Caches using .by_item() and validates presence.
7. test_update_rapid_cache: Updates TTL to 48 hours with field mask and verifies state running.
8. test_disable_rapid_cache: Initiates disablement, polls LRO to completion, and verifies state disabled.
9. test_disable_rapid_cache_non_existent: Verifies Code::NotFound when disabling a non-existent cache.
   Includes purge_rapid_caches cleanup helper to ensure test buckets can be cleanly deleted without leaking cloud resources."