MM-40813: Add config interface (#19571)

We create the basic boilerplate to pass services
to each product. The basic idea is to have a map
containing service names and the services.

The server will pass all services to all products.
But it is on the product, to choose what services
they will actually consume.

Each product will cast the received service to
an interface with the methods that product requires.

https://mattermost.atlassian.net/browse/MM-40813

```release-note
NONE
```

app/plugin_requests.go

@@ -118,7 +118,7 @@ func (ch *Channels) servePluginRequest(w http.ResponseWriter, r *http.Request, h
 	token := ""
 	context := &plugin.Context{
 		RequestId:      model.NewId(),
-		IPAddress:      utils.GetIPAddress(r, ch.srv.Config().ServiceSettings.TrustedProxyIPHeader),
+		IPAddress:      utils.GetIPAddress(r, ch.cfgSvc.Config().ServiceSettings.TrustedProxyIPHeader),
 		AcceptLanguage: r.Header.Get("Accept-Language"),
 		UserAgent:      r.UserAgent(),
 	}
@@ -183,7 +183,7 @@ func (ch *Channels) servePluginRequest(w http.ResponseWriter, r *http.Request, h
 					mlog.String("user_id", userID),
 				}
 
-				if *ch.srv.Config().ServiceSettings.ExperimentalStrictCSRFEnforcement {
+				if *ch.cfgSvc.Config().ServiceSettings.ExperimentalStrictCSRFEnforcement {
 					mlog.Warn(csrfErrorMessage, fields...)
 				} else {
 					mlog.Debug(csrfErrorMessage, fields...)
@@ -212,7 +212,7 @@ func (ch *Channels) servePluginRequest(w http.ResponseWriter, r *http.Request, h
 
 	params := mux.Vars(r)
 
-	subpath, _ := utils.GetSubpathFromConfig(ch.srv.Config())
+	subpath, _ := utils.GetSubpathFromConfig(ch.cfgSvc.Config())
 
 	newQuery := r.URL.Query()
 	newQuery.Del("access_token")