Add option to use PostPass OSM data

[spwoodcock]

Is your feature request related to a problem? Please describe.

• PostPass may become an official OSM project if it succeedes - allowing OSM data to be extracted using PostGIS query: https://github.com/woodpeck/postpass
• This package is used to extract data from the raw-data-api via HTTP request.
• We may as well add support for PostPass to this lib as an experimental feature.
• Adding this support would allow for including this plugin

Describe the solution you'd like

• Add a new get_postpass_data function that takes a filter config JSON using raw-data-api JSON syntax.
• This should be parsed into SQL. Both raw-data-api and postpass use osm2pgsql to store the data in the db, so we can reuse the logic here to build the SQL query: https://github.com/hotosm/raw-data-api/blob/develop/src/query_builder/builder.py
• In the end, the user should be able to provide a config JSON like those defined here to extract data from either raw-data-api, or via PostPass.
• The data for PostPass is always returned as a geojson, so hardcode that.

Usage

• This package would be used in a QGIS conflation plugin.
• It could also be used for all tools requiring data extracts from OpenStreetMap as part of their workflow.