RFC 8414 OAuth 2.0 Authorization Server Metadata Support

[vamsii777]

This PR introduces support for RFC 8414 Authorization Server Metadata, allowing OAuth clients to auto-discover server configurations.

Breaking Changes

1. OAuth2 Initialization Changes

The OAuth2 initializer now requires an issuer parameter and optionally supports jwksEndpoint for a custom JWKS URL.

• Old:

  OAuth2(codeManager: codeManager, tokenManager: tokenManager, ...)

• New (Breaking):

  OAuth2(
      issuer: "https://auth.example.com", // Required
      jwksEndpoint: "https://auth.example.com/.well-known/jwks.json", // Optional
      codeManager: codeManager,
      tokenManager: tokenManager,
      ...
  )

  Note: If jwksEndpoint is not provided, it defaults to {issuer}/.well-known/jwks.json. Ensure compliance with RFC 8414 if relying on the default endpoint.

2. New Required Endpoints

• Adds /.well-known/oauth-authorization-server endpoint, returning metadata per RFC 8414 with cache control headers.

New Features

1. Server Metadata Support

• Automatic metadata generation based on server configuration.
• Full support for RFC 8414 required fields.
• ServerMetadataProvider protocol for customized metadata handling.

2. Metadata Customization

• Customize metadata by implementing ServerMetadataProvider:

  protocol ServerMetadataProvider: Sendable {
      func getMetadata() async throws -> OAuthServerMetadata
  }

  Example:

  struct CustomMetadataProvider: ServerMetadataProvider {
      func getMetadata() async throws -> OAuthServerMetadata {
          // Return custom metadata
      }
  }

Migration Guide

1. Update OAuth2 Initialization to include the required issuer.

   let oauth = OAuth2(
       issuer: "https://auth.example.com",
       tokenManager: myTokenManager,
       clientRetriever: myClientRetriever,
       oAuthHelper: myOAuthHelper
   )

2. Optionally provide jwksEndpoint if using a custom JWKS endpoint.

   let oauth = OAuth2(
       issuer: "https://auth.example.com",
       jwksEndpoint: "https://custom.example.com/keys",
       ...
   )

3. Implement custom metadata with ServerMetadataProvider if needed:

   let oauth = OAuth2(
       issuer: "https://auth.example.com",
       ...
       metadataProvider: CustomMetadataProvider()
   )

Closes #11