MM-49984: Displaying active search backend in system console (#22721)

This will be shown in the system console to let users know
which search backend is active.

Implemented via adding an extra param in the ping response.

https://mattermost.atlassian.net/browse/MM-49984

```release-note
The database section in the system console now has an additional
read-only section which shows the active search backend in use.

This can be helpful to confirm which is the currently active
search engine when there are multiple of them configured.
```
---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/api4/system.go

@@ -190,6 +190,8 @@ func getSystemPing(c *Context, w http.ResponseWriter, r *http.Request) {
 		s["CanReceiveNotifications"] = c.App.SendTestPushNotification(deviceID)
 	}
 
+	s["ActiveSearchBackend"] = c.App.ActiveSearchBackend()
+
 	if s[model.STATUS] != model.StatusOk {
 		w.WriteHeader(http.StatusInternalServerError)
 	}