Support retrieving downsampled time series for backfilling
As seen in #175 (comment 1870036944), we'd need an option to backfill data, even if Thanos has already compacted and downsampled it. This is currently not possible, because Tamland requests 1h resolution data.
With Thanos, we can use https://thanos.io/v0.6/components/query/#auto-downsampling to still get data. However, Tamland aims to support Prometheus and should not become specific to Thanos.
What we can do here is implement a change similar to #175 (comment 1870036944), to allow the execution environment to specify both headers and query parameters with specific needs.
The query parameters can come from the "header provider" script we call or, alternatively, we can also use a ENV variable for this particular setting (easier, but less clean).