Move health check to the backend (#2120)

This PR moves the health check to backend only leaving in the frontend
the functionality to test the dbconnector datasource.

Leaving the `dbconnector.testDataSource` should be fine since the
datasource types we allow for db connection with Zabbix already are
backend datasources, and so their health requests would go through the
backend.

Verified:
Clicking test and seeing a `health` request go out.

IMPORTANT: While testing this in the UI, I found a bug with the config
editor - whenever a change is made in the UI and tested, the changes
don't take effect (i.e. disabling trends, keeps `trends` set to `true`,
enabling db connection keep `dbConnectionEnabled` set to `false` and so
on.). Created a separate
[issue](https://github.com/orgs/grafana/projects/457/views/40?pane=issue&itemId=3627315751&issue=grafana%7Coss-big-tent-squad%7C132)
to fix this

Fixes https://github.com/grafana/oss-big-tent-squad/issues/124
Fixes https://github.com/grafana/grafana-zabbix/issues/2004

.changeset/orange-owls-tie.md

@@ -0,0 +1,5 @@
+---
+'grafana-zabbix': minor
+---
+
+Moves health check to the backend