mcp/examples: moves middleware examples into example folder (#356)

For better organization.

examples/client/middleware/main.go

@@ -0,0 +1,30 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"context"
+	"sync/atomic"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+var nextProgressToken atomic.Int64
+
+// This middleware function adds a progress token to every outgoing request
+// from the client.
+func main() {
+	c := mcp.NewClient(&mcp.Implementation{Name: "test"}, nil)
+	c.AddSendingMiddleware(addProgressToken[*mcp.ClientSession])
+}
+
+func addProgressToken[S mcp.Session](h mcp.MethodHandler) mcp.MethodHandler {
+	return func(ctx context.Context, method string, req mcp.Request) (result mcp.Result, err error) {
+		if rp, ok := req.GetParams().(mcp.RequestParams); ok {
+			rp.SetProgressToken(nextProgressToken.Add(1))
+		}
+		return h(ctx, method, req)
+	}
+}