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/app/searchengine.go

@@ -60,3 +60,7 @@ func (a *App) PurgeBleveIndexes() *model.AppError {
 	}
 	return nil
 }
+
+func (a *App) ActiveSearchBackend() string {
+	return a.ch.srv.platform.SearchEngine.ActiveEngine()
+}