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_test.go

@@ -0,0 +1,58 @@
+package rpcplugin
+
+import (
+	"context"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mattermost/platform/plugin/plugintest"
+)
+
+func TestMain(t *testing.T) {
+	dir, err := ioutil.TempDir("", "")
+	require.NoError(t, err)
+	defer os.RemoveAll(dir)
+
+	plugin := filepath.Join(dir, "plugin")
+	compileGo(t, `
+		package main
+
+		import (
+			"github.com/mattermost/platform/plugin"
+			"github.com/mattermost/platform/plugin/rpcplugin"
+		)
+
+		type MyPlugin struct {}
+
+		func (p *MyPlugin) OnActivate(api plugin.API) error {
+			return nil
+		}
+
+		func (p *MyPlugin) OnDeactivate() error {
+			return nil
+		}
+
+		func main() {
+			rpcplugin.Main(&MyPlugin{})
+		}
+	`, plugin)
+
+	p, ipc, err := NewProcess(context.Background(), plugin)
+	require.NoError(t, err)
+	defer p.Wait()
+
+	muxer := NewMuxer(ipc, false)
+	defer muxer.Close()
+
+	var api plugintest.API
+
+	hooks, err := ConnectMain(muxer)
+	require.NoError(t, err)
+	assert.NoError(t, hooks.OnActivate(&api))
+	assert.NoError(t, hooks.OnDeactivate())
+}