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

pkg/datasource/datasource.go

@@ -3,6 +3,7 @@ package datasource
 import (
 	"context"
 	"errors"
+	"fmt"
 
 	"github.com/alexanderzobnin/grafana-zabbix/pkg/httpclient"
 	"github.com/alexanderzobnin/grafana-zabbix/pkg/metrics"
@@ -99,7 +100,7 @@ func (ds *ZabbixDatasource) CheckHealth(ctx context.Context, req *backend.CheckH
 	}
 
 	res.Status = backend.HealthStatusOk
-	res.Message = message
+	res.Message = fmt.Sprintf("Zabbix API version %s", message)
 	return res, nil
 }