MM-10703 Adding blank request context to plugin hooks for future use. (#9043)

* Adding blank request context to plugin hooks for future use.

* Rename RequestContext to Context

* Adding context to ServeHTTP and ExecuteCommand

* Fixing import cycle in test.

app/plugin_test.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/gorilla/mux"
 	"github.com/mattermost/mattermost-server/model"
+	"github.com/mattermost/mattermost-server/plugin"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -68,7 +69,7 @@ func TestHandlePluginRequest(t *testing.T) {
 	var assertions func(*http.Request)
 	router := mux.NewRouter()
 	router.HandleFunc("/plugins/{plugin_id:[A-Za-z0-9\\_\\-\\.]+}/{anything:.*}", func(_ http.ResponseWriter, r *http.Request) {
-		th.App.servePluginRequest(nil, r, func(_ http.ResponseWriter, r *http.Request) {
+		th.App.servePluginRequest(nil, r, func(_ *plugin.Context, _ http.ResponseWriter, r *http.Request) {
 			assertions(r)
 		})
 	})