Implement proxy to back store API by Prometheus servers

[fabxc]

Since the most recent data is on Prometheus servers, we must be able to fan out to those for queries. For overall simplicity of the system it is feasible to have the sidecar implement the store API on top of Prometheus's remote read API or HTTP v1 API. Probably either works fine but the HTTP v1 API has proper stability guarantees.

[fabxc]

Initial implementation and testing of this is done. Optimisations might be required in the future but will get separate issues.