[MM-16473] Make plugins' ServerHTTP http.ResponseWriter hijackable (#14822)

* Make plugins' ServerHTTP http.ResponseWriter hijackable

* Rename brw to align with docs

* Fix error handling

plugin/http.go

@@ -4,18 +4,27 @@
 package plugin
 
 import (
+	"bufio"
 	"errors"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"net/rpc"
 
 	"github.com/mattermost/mattermost-server/v5/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 {