mcp: Allow registration of custom JSON-RPC methods (#956)

## 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

mcp/shared.go

@@ -727,6 +727,22 @@ type Params interface {
 	isNil() bool
 }
 
+// ParamsBase can be embedded in custom parameter structs to satisfy the
+// [Params] interface. It provides the required [Meta] field and the unexported
+// isParams marker method.
+//
+//	type SearchParams struct {
+//	    mcp.ParamsBase
+//	    Query string `json:"query"`
+//	}
+type ParamsBase struct {
+	Meta `json:"_meta,omitempty"`
+}
+
+func (*ParamsBase) isParams() {}
+
+func (p *ParamsBase) isNil() bool { return p == nil }
+
 // RequestParams is a parameter (input) type for an MCP request.
 type RequestParams interface {
 	Params
@@ -751,6 +767,20 @@ type Result interface {
 	SetMeta(map[string]any)
 }
 
+// ResultBase can be embedded in custom result structs to satisfy the
+// [Result] interface. It provides the required [Meta] field and the unexported
+// isResult marker method.
+//
+//	type SearchResult struct {
+//	    mcp.ResultBase
+//	    Hits []string `json:"hits"`
+//	}
+type ResultBase struct {
+	Meta `json:"_meta,omitempty"`
+}
+
+func (*ResultBase) isResult() {}
+
 // emptyResult is returned by methods that have no result, like ping.
 // Those methods cannot return nil, because jsonrpc2 cannot handle nils.
 type emptyResult struct{}