feat: ability to update credentials on long running client

[ldetmer]

This feature introduces the ability to change Service Account credentials on a long running client without having to restart the client.

Usage:

1. Create a MutableCredentials instance wrapping initial Service Account Credentials.
2. Create a client with MutableCredentials created in step 1
3. Whenever necessary call MutableCredentials.updateCredentials with updated credentials. The next API call on the client will utilize the new credentials.

[gemini-code-assist]

Summary of Changes

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This pull request enhances the Spanner client library by providing a mechanism to update authentication credentials dynamically within a running application. This is particularly beneficial for long-lived services that need to refresh or change their service account credentials without incurring downtime or complex client re-initialization logic, thereby improving security posture and operational flexibility.

Highlights

• Dynamic Credential Updates: Introduced a new MutableCredentials class that allows for the dynamic updating or rotation of ServiceAccountCredentials for long-running Spanner client instances without requiring client re-initialization.
• Credential Delegation: The MutableCredentials class acts as a wrapper, delegating all authentication-related operations (like getAuthenticationType, getRequestMetadata, and refresh) to an internal, scoped ServiceAccountCredentials instance.
• Comprehensive Testing: Added both unit tests (MutableCredentialsTest) and integration tests (ITMutableCredentialsTest) to thoroughly validate the functionality and behavior of the new mutable credentials, including scenarios for successful authentication and subsequent failure after updating to invalid credentials.

Changelog

• google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/MutableCredentials.java
  • Added a new class MutableCredentials to enable dynamic updates of ServiceAccountCredentials for Spanner clients.
• google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/ITMutableCredentialsTest.java
  • Added an integration test to verify the MutableCredentials functionality in a real Spanner client environment, demonstrating credential updates and their impact on authorization.
• google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/MutableCredentialsTest.java
  • Added a unit test for MutableCredentials to ensure proper delegation and updating of underlying ServiceAccountCredentials using mock objects.

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[gemini-code-assist]

Code Review

This pull request introduces a "MutableCredentials" class to enable dynamic updating of service account credentials for long-running Spanner clients, facilitating credential rotation without client re-initialization. However, the current implementation has thread-safety issues and exposes mutable internal state, which could lead to security vulnerabilities in a multi-threaded environment. Specifically, the lack of "volatile" on the "delegate" field and the exposure of the mutable "scopes" list need to be addressed. While the overall structure is good and includes tests, these security concerns are critical.