Comprehensive Refactoring of OAuth Components for Concurrency and PKCE Support

[vamsii777]

This Pull Request encompasses a series of refactoring efforts aimed at enhancing the OAuth components. Key changes include:

• Code Cleanup and Documentation: Removed redundant comments and updated documentation for CodeManager, ensuring clarity and maintainability.
• Enhanced Sendable Compatibility: Updated OAuthClient properties to support Sendable protocol, aligning with Swift's concurrency model.
• Concurrent Access in StaticClientRetriever: Refactored StaticClientRetriever for safe concurrent access, enhancing performance and reliability in multi-threaded environments.
• OAuthHelper Refinements: Multiple updates in OAuthHelper+remote.swift, including refactoring of async methods for better asynchronous handling.
• Code Streamlining: Eliminated unnecessary file headers and imports, streamlining the codebase.
• OAuthFlowType Enhancement: Added Sendable conformance to OAuthFlowType, further aligning with concurrency requirements.
• PKCE Support: Introduced PKCEValidator for robust Proof Key for Code Exchange (PKCE) validation.
• Nonce Parameter Integration: Added nonce parameter to the generateCode function and corresponding test methods, enhancing security measures in the code generation process.
• Token Management: Extended the TokenManager protocol with new token generation methods, augmenting its functionality.

These collective changes are geared towards improving the robustness, security, and maintainability of OAuth implementation, ensuring it meets modern standards of concurrency and security protocols like PKCE.