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_test.go

@@ -0,0 +1,57 @@
+package rpcplugin
+
+import (
+	"encoding/json"
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/mock"
+
+	"github.com/mattermost/platform/plugin"
+	"github.com/mattermost/platform/plugin/plugintest"
+)
+
+func testAPIRPC(api plugin.API, f func(plugin.API)) {
+	r1, w1 := io.Pipe()
+	r2, w2 := io.Pipe()
+
+	c1 := NewMuxer(NewReadWriteCloser(r1, w2), false)
+	defer c1.Close()
+
+	c2 := NewMuxer(NewReadWriteCloser(r2, w1), true)
+	defer c2.Close()
+
+	id, server := c1.Serve()
+	go ServeAPI(api, server, c1)
+
+	remote := ConnectAPI(c2.Connect(id), c2)
+	defer remote.Close()
+
+	f(remote)
+}
+
+func TestAPI(t *testing.T) {
+	var api plugintest.API
+	defer api.AssertExpectations(t)
+
+	type Config struct {
+		Foo string
+		Bar struct {
+			Baz string
+		}
+	}
+
+	api.On("LoadPluginConfiguration", mock.MatchedBy(func(x interface{}) bool { return true })).Run(func(args mock.Arguments) {
+		dest := args.Get(0).(interface{})
+		json.Unmarshal([]byte(`{"Foo": "foo", "Bar": {"Baz": "baz"}}`), dest)
+	}).Return(nil)
+
+	testAPIRPC(&api, func(remote plugin.API) {
+		var config Config
+		assert.NoError(t, remote.LoadPluginConfiguration(&config))
+
+		assert.Equal(t, "foo", config.Foo)
+		assert.Equal(t, "baz", config.Bar.Baz)
+	})
+}