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/conformance_test.go

@@ -135,7 +135,7 @@ func runServerTest(t *testing.T, test *conformanceTest) {
 	// Connect the server, and connect the client stream,
 	// but don't connect an actual client.
 	cTransport, sTransport := NewInMemoryTransports()
-	ss, err := s.Connect(ctx, sTransport)
+	ss, err := s.Connect(ctx, sTransport, nil)
 	if err != nil {
 		t.Fatal(err)
 	}