Authorization Server: customize authorization code request validation

[arnabnandy7]

Summary

• Add an authorization code request validator hook to McpAuthorizationServerConfigurer
• Apply custom validators using the existing object post-processor pattern
• Allow applications to support CIMD clients that use dynamic localhost redirect URI ports
• Preserve Spring Authorization Server’s strict validator as the default
• Document configuration and security considerations
• Add test coverage for custom redirect URI validation

Fixes #84

Testing

./mvnw -pl mcp-authorization-server -Dtest=McpAuthorizationServerConfigurerTest test