[google_cloud] Add project ID discovery from credentials JSON file

# Add project ID discovery from credentials JSON file ## Description ### Problem `projectIdFromEnvironment()` only checks environment variable values from the `gcpProjectIdEnvironmentVariables` list. It does not read the `project_id` field from the JSON file pointed to by the `GOOGLE_APPLICATION_CREDENTIALS` environment variable. This means that when developers use a service account JSON file for authentication, the project ID from that file is ignored, requiring them to set an additional environment variable. ### Current Implementation ```dart String? projectIdFromEnvironment() { for (var envKey in gcpProjectIdEnvironmentVariables) { final value = Platform.environment[envKey]; if (value != null) return value; } return null; } ``` ### Proposed Solution Add a new function `projectIdFromCredentialsFile()` that reads the credentials file: ```dart String? projectIdFromCredentialsFile() { final path = Platform.environment['GOOGLE_APPLICATION_CREDENTIALS']; if (path == null) return null; try { final json = jsonDecode(File(path).readAsStringSync()); return json['project_id'] as String?; } catch (e) { // If file doesn't exist or is invalid, return null return null; } } ``` Then update `computeProjectId()` to check this source: ```dart Future<String> computeProjectId() async { // 1. Check environment variables final envValue = projectIdFromEnvironment(); if (envValue != null) return envValue; // 2. Check credentials file final credentialsValue = projectIdFromCredentialsFile(); if (credentialsValue != null) return credentialsValue; // 3. Check metadata server return await projectIdFromMetadataServer(); } ``` ### Use Case This provides a more seamless developer experience: ```dart // Before: Need both credentials file AND project ID env var export GOOGLE_APPLICATION_CREDENTIALS="/path/to/service-account.json" export GCP_PROJECT="my-project-id" // Redundant! // After: Just credentials file is enough export GOOGLE_APPLICATION_CREDENTIALS="/path/to/service-account.json" // project_id is automatically read from the JSON file ``` ### References - Current implementation: [gcp_project.dart](https://github.com/invertase/functions-framework-dart/blob/main/google_cloud/lib/src/gcp_project.dart#L44-L51) - Related to replacing `googleapis_auth_utils` functionality