initial work for supporting token exchange rfc

[havetisyan]

Description

initial work to support token exchange rfc8693. All the token request handling is now moved to the AccessTokenRequest class including validating and parsing of any subject/actor/assertion/etc tokens in the request.

Contribution Checklist:

• The pull request does not introduce any breaking changes
• I have read the contribution guidelines.
• Create an issue and link to the pull request.

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[gemini-code-assist]

Summary of Changes

Hello @havetisyan, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request initiates the implementation of OAuth 2.0 Token Exchange (RFC 8693) within the system. The core change involves a significant refactoring of how token requests are handled, moving validation and parsing logic into a dedicated AccessTokenRequest class and centralizing token-related configurations into a new TokenConfigOptions object. While the TOKEN_EXCHANGE endpoint is currently a placeholder, these changes establish a robust foundation for its complete functionality.

Highlights

• Token Exchange RFC 8693 Support: This pull request introduces initial work to support the OAuth 2.0 Token Exchange (RFC 8693) specification, laying the groundwork for future implementation.
• Refactored Token Request Handling: All token request processing, including validation and parsing of subject, actor, and assertion tokens, has been centralized and moved into the AccessTokenRequest class.
• Centralized Token Configuration: A new TokenConfigOptions class has been introduced to encapsulate and manage configuration options related to token processing, such as KeyStore and JWT processors, improving modularity and maintainability.
• Simplified ZTSImpl Logic: The ZTSImpl class has been refactored to utilize the new TokenConfigOptions object and retrieve pre-validated token objects directly from AccessTokenRequest, reducing redundant validation logic within ZTSImpl.

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[gemini-code-assist]

Code Review

This pull request introduces initial support for RFC 8693 token exchange, refactoring token request handling into the AccessTokenRequest class. This is a solid architectural improvement that enhances encapsulation and modularity. The introduction of TokenConfigOptions is also a clean way to manage configuration. The changes are accompanied by a good set of new unit tests. I have a few suggestions to further improve the implementation and test quality.