v8.0 module release (#22975)

https://mattermost.atlassian.net/browse/MM-52079

```release-note
We upgrade the module version to 8.0. The new module path is github.com/mattermost-server/server/v8.
```


Co-authored-by: Doug Lauder <wiggin77@warpmail.net>

server/plugin/http.go

@@ -0,0 +1,103 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package plugin
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"net/rpc"
+
+	"github.com/mattermost/mattermost-server/server/v8/platform/shared/mlog"
+)
+
+type hijackedResponse struct {
+	conn    net.Conn
+	bufrw   *bufio.ReadWriter
+	readBuf []byte
+}
+
+type httpResponseWriterRPCServer struct {
+	w   http.ResponseWriter
+	log *mlog.Logger
+	hjr *hijackedResponse
+}
+
+func (w *httpResponseWriterRPCServer) Header(args struct{}, reply *http.Header) error {
+	*reply = w.w.Header()
+	return nil
+}
+
+func (w *httpResponseWriterRPCServer) Write(args []byte, reply *struct{}) error {
+	_, err := w.w.Write(args)
+	return err
+}
+
+func (w *httpResponseWriterRPCServer) WriteHeader(args int, reply *struct{}) error {
+	// Check if args is a valid http status code. This prevents plugins from crashing the server with a panic.
+	// This is a copy of the checkWriteHeaderCode function in net/http/server.go in the go source.
+	if args < 100 || args > 999 {
+		w.log.Error(fmt.Sprintf("Plugin tried to write an invalid http status code: %v. Did not write the invalid header.", args))
+		return errors.New("invalid http status code")
+	}
+	w.w.WriteHeader(args)
+	return nil
+}
+
+func (w *httpResponseWriterRPCServer) SyncHeader(args http.Header, reply *struct{}) error {
+	dest := w.w.Header()
+	for k := range dest {
+		if _, ok := args[k]; !ok {
+			delete(dest, k)
+		}
+	}
+	for k, v := range args {
+		dest[k] = v
+	}
+	return nil
+}
+
+type httpResponseWriterRPCClient struct {
+	client *rpc.Client
+	header http.Header
+}
+
+var _ http.ResponseWriter = (*httpResponseWriterRPCClient)(nil)
+
+func (w *httpResponseWriterRPCClient) Header() http.Header {
+	if w.header == nil {
+		w.client.Call("Plugin.Header", struct{}{}, &w.header)
+	}
+	return w.header
+}
+
+func (w *httpResponseWriterRPCClient) Write(b []byte) (int, error) {
+	if err := w.client.Call("Plugin.SyncHeader", w.header, nil); err != nil {
+		return 0, err
+	}
+	if err := w.client.Call("Plugin.Write", b, nil); err != nil {
+		return 0, err
+	}
+	return len(b), nil
+}
+
+func (w *httpResponseWriterRPCClient) WriteHeader(statusCode int) {
+	if err := w.client.Call("Plugin.SyncHeader", w.header, nil); err != nil {
+		return
+	}
+	w.client.Call("Plugin.WriteHeader", statusCode, nil)
+}
+
+func (w *httpResponseWriterRPCClient) Close() error {
+	return w.client.Close()
+}
+
+func connectHTTPResponseWriter(conn io.ReadWriteCloser) *httpResponseWriterRPCClient {
+	return &httpResponseWriterRPCClient{
+		client: rpc.NewClient(conn),
+	}
+}