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>

webapp/platform/client/src/client4.ts

@@ -2439,6 +2439,7 @@ export default class Client4 {
     ping = () => {
         return this.doFetch<{
             status: string;
+            ActiveSearchBackend: string;
         }>(
             `${this.getBaseRoute()}/system/ping?time=${Date.now()}`,
             {method: 'get'},