Add support for thanos querier as the prometheus url

[RoryDoherty]

Is your feature request related to a problem? Add support for thanos querier as the prometheus source

Describe the solution you'd like In the global_config I should be able to set the prometheus_url to the thanos querier service and robusta would be able to query it for information relating to the alert

Describe alternatives you've considered I have tried adding a service for the prometheus instances and using that however all of our other services use thanos querier without any issue so it would be better if Robusta could also make use of it

Additional context There is a thread in the slack channel regarding this