Support custom method handlers with custom params and return types on MCPServer

[maxisbey]

The SDK should provide a clear, documented way to register custom JSON-RPC method handlers on the server side that can:

1. Receive custom parameter types — not just the built-in MCP request types, but arbitrary structured params (dicts, Pydantic models, etc.)
2. Return custom response types — not just the built-in MCP result types

This is important for:

• Protocol extensions and experimentation
• Custom server-to-server communication patterns
• Future spec features (e.g., MRTR) that may introduce new method/response types

The low-level server was recently refactored to use on_* constructor kwargs (PR #1985), which gives a cleaner handler registration pattern. This issue is about making sure there is an explicit, documented escape hatch for methods that are not part of the core MCP spec.

Open questions

• Should custom handlers go through the same on_* pattern, or a separate register_method() API?
• How do custom params get validated/parsed?
• How does this interact with middleware (Middleware Support in MCP #1233)?

Ref: V2 planning discussion