[core] use client interceptor for adding auth token in c++ client calls

[sampan-s-nayak]

Description

• Use client interceptor for adding auth tokens in grpc calls when AUTH_MODE=token
• BuildChannel() will automatically include the interceptor
• Removed auth_token parameter from ClientCallImpl
• removed manual auth from python_gcs_subscriber.cc
• tests to verify auth works for autoscaller apis

[gemini-code-assist]

Code Review

This pull request introduces a robust token-based authentication mechanism for the Ray dashboard UI and gRPC communications. The implementation is well-designed, leveraging gRPC interceptors for both client and server to transparently handle token injection and validation. This is a clean approach that avoids manual token handling in every RPC call. The dashboard's HTTP server is also correctly updated with middleware to protect endpoints, while whitelisting necessary public paths for the UI to function. The frontend changes, including the authentication dialog and global error handling via an axios interceptor, are solid. The addition of comprehensive unit, integration, and frontend tests is commendable. I have one suggestion to reduce code duplication in the Python gRPC client interceptor.