Is your feature request related to a problem? Please describe.

I'm contributing about SEP-2624 in modelcontextprotocol/experimental-ext-interceptors, which implements interceptors for MCP protocols. This feature requires extending the protocol with custom JSON-RPC methods.

Although the Golang SDK provides a solid interface, it currently does not allow to extend handlers for custom JSON-RPC methods.

Describe the solution you'd like

The mcp package can expose following interfaces

• Add methods to both the Client and Server to register receiving and sending custom methods. For example:
  • AddReceivingCustomMethod[P paramsPtr[T], R Result, T any](method string, handler typedServerMethodHandler[P, R])
  • AddSendingCustomMethod[P paramsPtr[T], R Result, T any](method string, handler typedClientMethodHandler[P, R])
• Export other internal components that are necessary to implement these custom handlers, such as Params.isParams, Result.isResult, typedServerMethodHandler, typedClientMethodHandler, paramsPtr, etc.

Describe alternatives you've considered

N/A

Additional context

For reference, we have already implemented AddReceivingCustomMethod for the server in a vendored version of the library. You can view the implementation details here:

• AddReceivingCustomMethod on the server
• merging part

happy to contribute once agreed.