add alertmanager alerts/group alerts routes

[domasx2]

adds endpoints to get alertmanager grouped & ungrouped alerts. basically v2/alerts and /v2/alerts/groups from alertmanager api

[owen-d]

I don't think we should include the v2 API as cortex/loki don't currently implement them. We can use the /v1/{rules,alerts} instead in the meantime, yes?

[domasx2]

I don't think we should include the v2 API as cortex/loki don't currently implement them. We can use the /v1/{rules,alerts} instead in the meantime, yes?

These are alertmanager endpoints, whilst /v1/{rules,alerts} are prometheus endpoints. I think we need the ones in this PR to achieve feature parity. What we might not need is prometheus /v1/alerts... I'll have a nother think tomorrow about it.

Maybe it would be less confusing to do what cortex does and prefix with prom or alertmanager depending on which system the endpoint "belongs" to? And also probably will need to add datasource id to all paths