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

* Make plugins' ServerHTTP http.ResponseWriter hijackable

* Rename brw to align with docs

* Fix error handling

app/plugin_api_tests/manual.test_http_hijack_plugin/main.go

@@ -0,0 +1,35 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package main
+
+import (
+	"net/http"
+
+	"github.com/mattermost/mattermost-server/v5/plugin"
+)
+
+type Plugin struct {
+	plugin.MattermostPlugin
+}
+
+func (p *Plugin) ServeHTTP(c *plugin.Context, w http.ResponseWriter, r *http.Request) {
+	hj, ok := w.(http.Hijacker)
+	if !ok {
+		w.WriteHeader(http.StatusInternalServerError)
+		return
+	}
+
+	conn, brw, err := hj.Hijack()
+	if conn == nil || brw == nil || err != nil {
+		w.WriteHeader(http.StatusInternalServerError)
+		return
+	}
+
+	conn.Write([]byte("HTTP/1.1 200\n\nOK"))
+	conn.Close()
+}
+
+func main() {
+	plugin.ClientMain(&Plugin{})
+}