Auth: implement OAuth2 Client Credentials flow identity provider

[rayl15]

Description

This PR implements the OAuth2 Client Credentials flow identity provider as requested in issue #400. The implementation provides both TokenProvider and Verifier traits support with comprehensive token caching and error handling.

Core Implementation

• OAuth2TokenProvider: Implements TokenProvider trait with client_id/client_secret authentication
• OAuth2Verifier: Implements Verifier trait leveraging existing JWT infrastructure
• Thread-safe token caching with configurable buffer time for automatic renewal
• Form-encoded requests following RFC 6749 OAuth2 standard
• Integration with existing JwtBuilder pattern for seamless adoption

Files Modified

• data-plane/core/auth/src/oauth2.rs - New OAuth2 implementation (704 lines)
• data-plane/core/auth/src/errors.rs - Added OAuth2-specific error types
• data-plane/core/auth/src/lib.rs - Added module exports for easy access
• data-plane/core/auth/src/builder.rs - Extended builder pattern for OAuth2 support

Testing

All tests pass with comprehensive coverage:

cargo test oauth2 --lib
# running 13 tests
# test result: ok. 13 passed; 0 failed; 0 ignored

Test scenarios include:

• Token acquisition and caching
• Error handling (401, 500, timeouts, malformed responses)
• Token expiration and renewal
• Concurrent request handling
• Configuration validation

Usage Example

use slim_auth::{OAuth2ClientCredentialsConfig, OAuth2TokenProvider, TokenProvider};

let config = OAuth2ClientCredentialsConfig {
    client_id: "my-client".to_string(),
    client_secret: "my-secret".to_string(),
    token_endpoint: "https://auth.example.com/oauth/token".to_string(),
    scope: Some("api:read".to_string()),
    audience: Some("https://api.example.com".to_string()),
    cache_buffer: Some(Duration::from_secs(300)),
    timeout: Some(Duration::from_secs(30)),
};

let provider = OAuth2TokenProvider::new(config)?;
let token = provider.get_token_async().await?;

Type of Change

• Bugfix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other (please describe)

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass

Fixes: #400

[amitami2]

@rayl15 there is already an opened PR for this :
#464