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/example_middleware_test.go

@@ -114,10 +114,10 @@ func Example_loggingMiddleware() {
 	ctx := context.Background()
 
 	// Connect server and client
-	serverSession, _ := server.Connect(ctx, serverTransport)
+	serverSession, _ := server.Connect(ctx, serverTransport, nil)
 	defer serverSession.Close()
 
-	clientSession, _ := client.Connect(ctx, clientTransport)
+	clientSession, _ := client.Connect(ctx, clientTransport, nil)
 	defer clientSession.Close()
 
 	// Call the tool to demonstrate logging