mcp: Allow registration of custom JSON-RPC methods

[guglielmo-san]

Description

This PR introduces support for custom (non-standard) JSON-RPC methods in the MCP Go SDK.
This feature enables servers to handle arbitrary JSON-RPC methods and clients to invoke them, all while maintaining full type safety and integrating seamlessly with the SDK's existing middleware and session management.

• Server-Side Registration: Added AddReceivingCustomMethod to allow servers to register handlers for custom methods. Incoming requests are automatically unmarshaled into user-defined parameter structs.
• Client-Side Calling: Added AddSendingCustomMethod which returns a strongly-typed caller function that clients can use to invoke custom methods and receive strongly-typed results.
• Helper Structs: Provided ParamsBase and ResultBase for users to embed in their custom structs. This allows them to easily satisfy the SDK's internal Params and Result interfaces.

Fixes #954