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

* Moving plugins to use hashicorp go-plugin.

* Tweaks from feedback.

plugin/io_rpc.go

@@ -0,0 +1,63 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package plugin
+
+import (
+	"bufio"
+	"encoding/binary"
+	"io"
+)
+
+type rwc struct {
+	io.ReadCloser
+	io.WriteCloser
+}
+
+func (rwc *rwc) Close() (err error) {
+	err = rwc.WriteCloser.Close()
+	if rerr := rwc.ReadCloser.Close(); err == nil {
+		err = rerr
+	}
+	return
+}
+
+func NewReadWriteCloser(r io.ReadCloser, w io.WriteCloser) io.ReadWriteCloser {
+	return &rwc{r, w}
+}
+
+type RemoteIOReader struct {
+	conn io.ReadWriteCloser
+}
+
+func (r *RemoteIOReader) Read(b []byte) (int, error) {
+	var buf [10]byte
+	n := binary.PutVarint(buf[:], int64(len(b)))
+	if _, err := r.conn.Write(buf[:n]); err != nil {
+		return 0, err
+	}
+	return r.conn.Read(b)
+}
+
+func (r *RemoteIOReader) Close() error {
+	return r.conn.Close()
+}
+
+func ConnectIOReader(conn io.ReadWriteCloser) io.ReadCloser {
+	return &RemoteIOReader{conn}
+}
+
+func ServeIOReader(r io.Reader, conn io.ReadWriteCloser) {
+	cr := bufio.NewReader(conn)
+	defer conn.Close()
+	buf := make([]byte, 32*1024)
+	for {
+		n, err := binary.ReadVarint(cr)
+		if err != nil {
+			break
+		}
+		if written, err := io.CopyBuffer(conn, io.LimitReader(r, n), buf); err != nil || written < n {
+			break
+		}
+	}
+}