PLT-7407: Back-end plugin mechanism (#7177)

* begin backend plugin wip

* flesh out rpcplugin. everything done except for minor supervisor stubs

* done with basic plugin infrastructure

* simplify tests

* remove unused test lines

plugin/rpcplugin/main.go

@@ -0,0 +1,46 @@
+package rpcplugin
+
+import (
+	"bufio"
+	"encoding/binary"
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/mattermost/platform/plugin"
+)
+
+// Makes a set of hooks available via RPC. This function never returns.
+func Main(hooks plugin.Hooks) {
+	ipc, err := InheritedProcessIPC()
+	if err != nil {
+		log.Fatal(err.Error())
+	}
+	muxer := NewMuxer(ipc, true)
+	id, conn := muxer.Serve()
+	buf := make([]byte, 11)
+	buf[0] = 0
+	n := binary.PutVarint(buf[1:], id)
+	if _, err := muxer.Write(buf[:1+n]); err != nil {
+		log.Fatal(err.Error())
+	}
+	ServeHooks(hooks, conn, muxer)
+	os.Exit(0)
+}
+
+// Returns the hooks being served by a call to Main.
+func ConnectMain(muxer *Muxer) (*RemoteHooks, error) {
+	buf := make([]byte, 1)
+	if _, err := muxer.Read(buf); err != nil {
+		return nil, err
+	} else if buf[0] != 0 {
+		return nil, fmt.Errorf("unexpected control byte")
+	}
+	reader := bufio.NewReader(muxer)
+	id, err := binary.ReadVarint(reader)
+	if err != nil {
+		return nil, err
+	}
+
+	return ConnectHooks(muxer.Connect(id), muxer), nil
+}