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