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_example_test.go

@@ -37,7 +37,7 @@ func ExampleSSEHandler() {
 	ctx := context.Background()
 	transport := mcp.NewSSEClientTransport(httpServer.URL, nil)
 	client := mcp.NewClient(&mcp.Implementation{Name: "test", Version: "v1.0.0"}, nil)
-	cs, err := client.Connect(ctx, transport)
+	cs, err := client.Connect(ctx, transport, nil)
 	if err != nil {
 		log.Fatal(err)
 	}