Bug: Links and Assets not sourced correctly from pgSTAC

[rajadain]

Description
pygeopapi expects links and assets to be top-level items, as specified in the STAC spec.

However, in pgSTAC, the collections and items schemas do not include them as top-level items, but store them in the contents JSONB column:

https://github.com/stac-utils/pgstac/blob/08ce5991e7840f51993d508c715fad44f889e5e5/src/pgstac/pgstac.sql#L889-L899

https://github.com/stac-utils/pgstac/blob/08ce5991e7840f51993d508c715fad44f889e5e5/src/pgstac/pgstac.sql#L2067-L2077

To access them when using a pgSTAC back-end, we should fetch from feature.properties.contents.links and feature.properties.contents.assets here:

pygeoapi/pygeoapi/api/stac.py

Lines 382 to 384 in 0c756e1

	for la in ['links', 'assets']:
	if feature.get(la) is None:
	feature[la] = []

Steps to Reproduce
Steps to reproduce the behavior:

• Setup pystac_client against a pygeoapi instance backed by pgSTAC
• Run search.item_collection()
• Observe the links and assets are incomplete and missing from the output:

Expected behavior
The links and assets are sourced correctly from the pgSTAC backend:

Environment

• OS: Linux via Docker running on macOS
• Python version: 12
• pygeoapi version: 0.23.4

[tomkralidis]

As discussed in #2340 (comment), a cleaner approach would be to write an (external) pgstac backend that does this transformation, and the core pygeoapi STAC/search code handles a STAC item proper.