support geo type mapping resolutions

[sgratzl]

coming from #389

e.g. query all counties of the state of California

possible mappings:

• nation -> state
• state -> county
• hrr -> state

[krivard]

this might be more easily implemented in delphi_utils.geomap, possibly as an extension of /variation on get_geo_values():

https://github.com/cmu-delphi/covidcast-indicators/blob/261b3a3868abd1527d8ac9867a8aca78f8ef46eb/_delphi_utils_python/delphi_utils/geomap.py#L559-L576

[krivard]

Now that cmu-delphi/covidcast-indicators#1159 has been merged, the following has to happen:

• @korlaxxalrok to pull the release lever in covidcast-indicators (no earlier than Monday 9 Aug)
• design a syntax for specifying contained/child regions as a URL parameter of a covidcast request
• implement in the API server by decoding the new syntax and applying delphi_utils.geomap.GeoMapper.get_geos_within()

I don't have any huge preferences on syntax, so long as we avoid the possibility where the same geo_id parameter value could be interpreted as multiple geo types. @sgratzl may have guidance?