Add attributes for aggregation areas and spatial joins

[panosatha]

Whenever we do a spatial join to add a new aggregation area we should save the geospatial file and the spatial join information.

This information could go in the settings.toml or a new text file with the name spatial_joins.toml. or something similar.

The field should include a list of aggregation for example:

[[aggregation]]
name = "Census Blockgroup"
file = "exposure/aggregation_areas/block_groups.gpkg"
field_name = "GEOID_short"

[[aggregation]]
name = "Subdivision"
file = "exposure/aggregation_areas/subdivision.gpkg"
field_name = "name"

We could do the same for any additional attributes.

We discussed with @Santonia27 that this should be a seperate toml file.