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/api.go

@@ -0,0 +1,62 @@
+package rpcplugin
+
+import (
+	"encoding/json"
+	"io"
+	"net/rpc"
+
+	"github.com/mattermost/platform/plugin"
+)
+
+type LocalAPI struct {
+	api   plugin.API
+	muxer *Muxer
+}
+
+func (h *LocalAPI) LoadPluginConfiguration(args struct{}, reply *[]byte) error {
+	var config interface{}
+	if err := h.api.LoadPluginConfiguration(&config); err != nil {
+		return err
+	}
+	b, err := json.Marshal(config)
+	if err != nil {
+		return err
+	}
+	*reply = b
+	return nil
+}
+
+type RemoteAPI struct {
+	client *rpc.Client
+	muxer  *Muxer
+}
+
+func ServeAPI(api plugin.API, conn io.ReadWriteCloser, muxer *Muxer) {
+	server := rpc.NewServer()
+	server.Register(&LocalAPI{
+		api:   api,
+		muxer: muxer,
+	})
+	server.ServeConn(conn)
+}
+
+var _ plugin.API = (*RemoteAPI)(nil)
+
+func (h *RemoteAPI) LoadPluginConfiguration(dest interface{}) error {
+	var config []byte
+	if err := h.client.Call("LocalAPI.LoadPluginConfiguration", struct{}{}, &config); err != nil {
+		return err
+	}
+	return json.Unmarshal(config, dest)
+}
+
+func (h *RemoteAPI) Close() error {
+	return h.client.Close()
+}
+
+func ConnectAPI(conn io.ReadWriteCloser, muxer *Muxer) *RemoteAPI {
+	return &RemoteAPI{
+		client: rpc.NewClient(conn),
+		muxer:  muxer,
+	}
+}