feat: add support for token-based authentication

[thisislvca]

Proposed changes

This PR adds proper support for temporary token authentication to the Deepgram Rust SDK. I'm working on a client-side application, and I was surprised to discover that the SDK only supported API key authentication with the "Token" prefix, thus not supporting token-based auth.

• API Keys: Should use the Token prefix (e.g., Token dg_xxx)
• Temporary Tokens: Should use the Bearer prefix (e.g., Bearer dg_xxx)

The existing implementation incorrectly used "Token" for all authentication, which worked for API keys but was wrong for temporary tokens.

Changes:

There are no breaking changes to current apps.

• Added AuthMethod enum to distinguish between API keys and temporary tokens
• Updated Deepgram struct to use the new authentication system (by default it'll still use API keys)
• Added Deepgram::with_temp_token() constructor for temporary token authentication
• Use new auth logic in websocket
• Added authentication documentation to README
• Added tests for the new authentication logic

All existing tests pass, and the change is backwards compatible for API usage patterns.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update or tests (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING.md doc
• I have added tests and/or examples that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

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[jcdyer]

Thanks for this contribution. It looks good. I don't think we want to commit to supporting RedactedString or AuthMethod as part of the public API at this point.

[thisislvca]

Good point, I initially made them public so I could extend it if I ever needed to, but it doesn't make sense at this stage you're right