Add boundaries to S2S database

[bpstewar]

Describe the solution you'd like
We need to provide users with access to the official World Bank boundaries for use in the aggregation process. This would require updating the database with the boundaries (ADM0, ADM1, and ADM2), and updating the API to allow for querying and extracting.

The API endpoint should deliver a geojson of the boundaries based on input ISO3 code and ADM level.

Describe alternatives you've considered
The boundaries will be available in other locations (ie - World Bank's Data Catalog), so this is not an absolute necessity.

Additional context
This will NOT require the API to do any aggregation in the backend, it is just a method to fetch the boundaries. I would expect the general process to look something like this (in Python)

import s2s_client as s2s

iso3 = 'KEN'
adm_level = 'ADM1'

# ken_boundaries should now be a GeoDataFrame
ken_boundaries = s2s.fetch_admin_boundaries(iso3, adm_level)

# fetch and summarize the h3 results for the desired geodaatframe
ken_population = s2s.get_summary(gdf, 'intersects', 'ntl', geometry=None)