Support alert provisioning

[GiorgioAresu]

It would be useful to provision alerts in a similar way to dashboards: files from external urls that get pulled from a sidecar when starting.

In our current setup we have one gitops repo per environment, and we try to keep repetition down to a minimum and simplify promotion across environments.
To do this, we already moved all the dashboards to a separate repo and we now use dashboardProvider, dashboards and downloadDashboards sections to reference those from there, using tags. A promotion now means updating a tag (downloadDashboards.env.GROUP_X_REF) instead of copy-pasting files across repos.
This can't currently be done with alerts, I think it would be good (and relatively easy) for them to behave the same.