Search is broken without the fields extension

[gadomski]

To reproduce

In one shell in the stac-fastapi root:

docker-compose run --service-ports --rm database

In a second shell:

ENABLED_EXTENSIONS=query python -m stac_fastapi.pgstac.app

Then:

$ curl http://localhost:8000/search
{"code":"AttributeError","description":"'SearchPostRequest' object has no attribute 'fields'"}%  

Expected behavior

stac-fastapi with the pgstac backend should allow search if the fields extension is not enabled. This does not affect the sqlalchemy backend, which appears to work w/o the fields extension.

Related issues

• Blocks my efforts to see if filter-lang parameter not being passed correctly to pgstac backend causing incorrect response for listing items in a collection #29 is fixed, because I can't enable query without fields (and apparently enabling fields fixes the issue)
• Might be fixed by changes in Align fields extension behavior with extension suggestions stac-fastapi#527, which makes our handling of the fields attribute on a search request more robust. We probably won't merge Align fields extension behavior with extension suggestions stac-fastapi#527, but whatever we do to resolve PgSTAC: Fields extension includes links even if not listed in includes #27 might help fix this issue