[google_cloud] Add caching to computeProjectId()

# Add caching to computeProjectId() ## Description ### Problem `computeProjectId()` makes fresh calls to environment variables and the metadata server on every invocation. This is inefficient when the project ID is requested multiple times during the application lifecycle. ### Current Implementation ```dart Future<String> computeProjectId() async { final localValue = projectIdFromEnvironment(); // Checks env vars every time if (localValue != null) { return localValue; } final result = await projectIdFromMetadataServer(); // Network call every time return result; } ``` While environment variable lookups are fast, metadata server queries involve network I/O and should be cached. ### Proposed Solution Add caching so discovery only happens once: ```dart String? _cachedProjectId; Future<String> computeProjectId() async { if (_cachedProjectId != null) { return _cachedProjectId!; } // ... existing discovery logic ... // Cache the result _cachedProjectId = result; return result; } /// Clears the cached project ID. /// /// This is primarily useful for testing scenarios where the project ID /// might change between tests. void clearProjectIdCache() { _cachedProjectId = null; } ``` ### Use Case Improves performance when project ID is accessed multiple times: ```dart // First call: performs discovery final projectId1 = await computeProjectId(); // Network call to metadata server // Subsequent calls: returns cached value final projectId2 = await computeProjectId(); // Instant final projectId3 = await computeProjectId(); // Instant // In tests: clear cache between tests clearProjectIdCache(); ``` ### Implementation Notes - The cache should be a simple module-level nullable variable - The cache should be checked first, before any discovery attempts - Provide a `clearProjectIdCache()` function for testing purposes - The cache is held for the lifetime of the Dart process (acceptable since project ID doesn't change at runtime) ### Testing Considerations Tests should call `clearProjectIdCache()` in `setUp()` or `tearDown()` to ensure test isolation: ```dart tearDown(() { clearProjectIdCache(); }); test('project ID from environment', () async { // Test will have clean slate }); ``` ### References - Current implementation: [gcp_project.dart](https://github.com/invertase/functions-framework-dart/blob/main/google_cloud/lib/src/gcp_project.dart#L26-L34) - Related to replacing `googleapis_auth_utils` functionality