feat: Add _meta support to MCPSchema (#368)

- Add _meta field to all MCP schema records for extensible metadata support
- Introduce Result and Notification sealed interfaces for better type organization
- Update Request interface to include SubscribeRequest and UnsubscribeRequest
- Add backwards-compatible constructors for existing record types
- Enhance test coverage for metadata field functionality

Breaking change: Modifyibng the Request seald inteface is a breaking change.

Author: pantanurag555