feat: Add OAuth 2.0 authentication support to Rust SDK

[huacnlee]

Summary

• Adds OAuth 2.0 as the recommended authentication method for the Rust SDK
• Implements new oauth feature with authorization flow support
• Updates signature logic to support both OAuth 2.0 (Bearer token) and legacy (HMAC) modes
• Provides comprehensive documentation and examples for OAuth usage

Key Changes

• New OAuth API: Config::from_oauth(client_id, access_token) constructor
• Feature-gated dependencies: oauth2, open, anyhow, url (behind oauth feature)
• Smart authentication: Automatically detects OAuth vs legacy mode based on token format
• No breaking changes: Full backward compatibility with existing API key authentication
• Documentation: Updated README with OAuth registration, authorization flow, and usage examples

Authentication Modes

1. OAuth 2.0 (Recommended): Uses Bearer tokens, no HMAC signatures required
2. Legacy API Key: Existing environment variable based authentication still supported

Test plan

• Verify OAuth feature compiles with cargo build --features oauth
• Test OAuth authorization flow with client registration
• Confirm Bearer token authentication works without HMAC signatures
• Validate legacy mode still works with environment variables
• Check backward compatibility (default build without oauth feature)

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