mcp: support stateless streamable sessions (#277)

Support stateless streamable sessions by adding a GetSessionID function
to StreamableHTTPOptions. If GetSessionID returns "", the session is
stateless, and no validation is performed. This is implemented by
providing the session a trivial initialization state.

To implement this, some parts of #232 (distributed sessions) are copied
over, since they add an API for creating an already-initialized session.

In total, the following new API is added:
- StreamableHTTPOptions.GetSessionID
- ServerSessionOptions (a new parameter to Server.Connect)
- ServerSessionState
- ClientSessionOptions (a new parameter to Client.Connect, for symmetry)

For #10

mcp/sse.go

@@ -221,7 +221,7 @@ func (h *SSEHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 		http.Error(w, "no server available", http.StatusBadRequest)
 		return
 	}
-	ss, err := server.Connect(req.Context(), transport)
+	ss, err := server.Connect(req.Context(), transport, nil)
 	if err != nil {
 		http.Error(w, "connection failed", http.StatusInternalServerError)
 		return