Support loading Google access token credentials

[tvainika]

If service account impersonation is used, then a plain access token is used for authentication.

[Copilot]

Pull Request Overview

This PR adds support for GCS access token credentials in addition to the existing service account credentials. The main changes enable the system to accept JSON credentials with a single access_token field, which will be parsed and used to create GoogleCredentials.

• Added Jackson databind dependency to parse JSON credentials
• Implemented access token credential detection and handling in CredentialsBuilder
• Added test coverage for access token credential flow
• Updated documentation to describe the new credential format support

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
storage/gcs/build.gradle	Added Jackson databind dependency for JSON parsing
storage/gcs/src/main/java/io/aiven/kafka/tieredstorage/storage/gcs/CredentialsBuilder.java	Refactored credential parsing to support access token format via new getCredentialsFromBytes method
storage/gcs/src/main/java/io/aiven/kafka/tieredstorage/storage/gcs/GcsStorageConfig.java	Updated configuration documentation to mention access token support
storage/gcs/src/test/java/io/aiven/kafka/tieredstorage/storage/gcs/ReloadableCredentialsProviderTest.java	Added test case for access token credentials and fixed private key string formatting
storage/gcs/src/test/java/io/aiven/kafka/tieredstorage/storage/gcs/GcsCredentialsAutoReloadIntegrationTest.java	Fixed private key string formatting for consistency
docs/configs.rst	Updated documentation to mention access token credential format

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.