Add license information to newly added files

Adds license information to the newly created `pkg/plugin/client/http2http.go` file.

- **License Addition**: Incorporates Apache License 2.0 information at the top of the `http2http.go` file, aligning with the repository's licensing practices for source files.

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For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/fatedier/frp/pull/4275?shareId=07b6614a-5cf5-4f9f-be18-0256c8aad313).

pkg/plugin/client/http2http.go

@@ -1,9 +1,22 @@
-// Package plugin contains various client plugins for frp
+// Copyright 2024 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package plugin
 
 import (
 	"io"
-    stdlog "log"
+	stdlog "log"
 	"net"
 	"net/http"
 	"net/http/httputil"