Move health check to the backed

[jcolladokuri]

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 to fix this

Fixes: #124

[jcolladokuri]

For this, I am using this approach for now, but it should be temporary, I have a separate issue to track migrating from DatasourceApi to DatasourceWithBackend when this is complete we will no longer need to have this here.

[itsmylife]

It looks good but we need to double check if we still need testDatasource()

[itsmylife]

Do we still need this method? I think this is needed for frontend implementation. Now the config editor must have called backend CheckHealth endpoint out of the box.

[jcolladokuri]

I thought we still needed it. Maybe once we migrate to DatasourceWithBackend (in progress) we don't, but even then, for this scenario we will still need it, because the part that tests the DB connection cannot be moved to the backend, in the frontend we call those datasource's "testDatasource" and they call their backend, and so here we merge the results from both.

I also checked a few other datasources and can see that they are still using the testDatasource method, for example: jaeger and tempo

[itsmylife]

They were frontend only data sources and migrated to backend. Actually I raised that question in some other place too. In prometheus we don't use it. Do you mind checking what happens when we remove that method?