Expose Public API to test SQL connections

Turribeach

We want to test database SQL connections in all our instances since our internal security policy is to change database passwords yearly and this invariably leads to some user database connections being missed, the password expiring, flows failing, data not being updated, etc. So we are trying to be more proactive and try to test connections programatically. I looked at the REST API and there isn't any available public APIs at the moment although I noticed the DSS GUI calls the /dip/api/admin/connections/test-sql internal API. So hopefully this means this private API can easily be exposed and be used by Dataiku administrators to test their database SQL connections are working.