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

* Moving plugins to use hashicorp go-plugin.

* Tweaks from feedback.

app/app.go

@@ -24,7 +24,7 @@ import (
 	tjobs "github.com/mattermost/mattermost-server/jobs/interfaces"
 	"github.com/mattermost/mattermost-server/mlog"
 	"github.com/mattermost/mattermost-server/model"
-	"github.com/mattermost/mattermost-server/plugin/pluginenv"
+	"github.com/mattermost/mattermost-server/plugin"
 	"github.com/mattermost/mattermost-server/store"
 	"github.com/mattermost/mattermost-server/store/sqlstore"
 	"github.com/mattermost/mattermost-server/utils"
@@ -41,10 +41,8 @@ type App struct {
 
 	Log *mlog.Logger
 
-	PluginEnv                *pluginenv.Environment
-	PluginConfigListenerId   string
-	IsPluginSandboxSupported bool
-	pluginStatuses           map[string]*model.PluginStatus
+	Plugins                *plugin.Environment
+	PluginConfigListenerId string
 
 	EmailBatching *EmailBatchingJob
 
@@ -231,8 +229,6 @@ func New(options ...Option) (outApp *App, outErr error) {
 		handlers: make(map[string]webSocketHandler),
 	}
 
-	app.initBuiltInPlugins()
-
 	return app, nil
 }