Move Channels into App (#18623)

* Move Channels into App

In this PR, we make Channels as part of App
instead of Server. This is part of the transition period
of moving fields from Server to Channels.

For now, Channels contains Server. So the hierarchy is

App -> Channels -> Server.

And as a first step, we also move httpService to Channels.

```release-note
NONE
```

* Fixing another test

```release-note
NONE
```

* new method

```release-note
NONE
```

app/plugin_test.go

@@ -342,7 +342,7 @@ func TestServePluginRequest(t *testing.T) {
 
 	w := httptest.NewRecorder()
 	r := httptest.NewRequest("GET", "/plugins/foo/bar", nil)
-	th.App.srv.ServePluginRequest(w, r)
+	th.App.ch.srv.ServePluginRequest(w, r)
 	assert.Equal(t, http.StatusNotImplemented, w.Result().StatusCode)
 }
 
@@ -386,7 +386,7 @@ func TestPrivateServePluginRequest(t *testing.T) {
 
 			request = mux.SetURLVars(request, map[string]string{"plugin_id": "id"})
 
-			th.App.srv.servePluginRequest(recorder, request, handler)
+			th.App.ch.srv.servePluginRequest(recorder, request, handler)
 		})
 	}
 
@@ -409,7 +409,7 @@ 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.srv.servePluginRequest(nil, r, func(_ *plugin.Context, _ http.ResponseWriter, r *http.Request) {
+		th.App.ch.srv.servePluginRequest(nil, r, func(_ *plugin.Context, _ http.ResponseWriter, r *http.Request) {
 			assertions(r)
 		})
 	})