Add optional subject delegation support for service account authentication

[varelaerick]

Add support for user impersonation via GOOGLE_ADS_SUBJECT environment variable when using service account credentials.

Changes:

• Add _get_subject() function to read subject from environment
• Enhance _create_credentials() to apply subject delegation for service accounts
• Maintain full backward compatibility - no changes to existing behavior
• Add logging for authentication method used

Usage:
Set GOOGLE_ADS_SUBJECT=user@example.com when using service account credentials with domain-wide delegation enabled.

Requirements:

• Service account credentials (not OAuth)
• Domain-wide delegation configured in Google Workspace Admin