MM-10702 Moving plugins to use hashicorp go-plugin. (#8978)

* Moving plugins to use hashicorp go-plugin.

* Tweaks from feedback.

plugin/http.go

@@ -0,0 +1,91 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package plugin
+
+import (
+	"io"
+	"net/http"
+	"net/rpc"
+)
+
+type HTTPResponseWriterRPCServer struct {
+	w http.ResponseWriter
+}
+
+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 {
+	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
+}
+
+func ServeHTTPResponseWriter(w http.ResponseWriter, conn io.ReadWriteCloser) {
+	server := rpc.NewServer()
+	server.Register(&HTTPResponseWriterRPCServer{
+		w: w,
+	})
+	server.ServeConn(conn)
+}
+
+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 (h *HTTPResponseWriterRPCClient) Close() error {
+	return h.client.Close()
+}
+
+func ConnectHTTPResponseWriter(conn io.ReadWriteCloser) *HTTPResponseWriterRPCClient {
+	return &HTTPResponseWriterRPCClient{
+		client: rpc.NewClient(conn),
+	}
+}