Support Application Default Credentials #289

[demoncoder-crypto]

Changes

• Added support for initializing the SDK without an API key using ADC
• Implemented token retrieval from ADC using the google-auth-library
• Added AuthOptions interface to configure ADC usage
• Updated RequestOptions to support ADC configuration
• Added comprehensive documentation and sample code
• Added tests for ADC functionality

Testing

• Added unit tests for ADC authentication flow
• Manually tested with both API key and ADC authentication

[google-cla]

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