PgSTAC: Fields "include" fails if collection not included

[duckontheweb]

Ported from stac-utils/stac-fastapi#394.

Original Issue:

See comment here: https://github.com/stac-utils/stac-fastapi/blob/81015a153c1d9f36d8e12f17a1bf67370396f472/stac_fastapi/pgstac/stac_fastapi/pgstac/core.py#L152

When using a fields "includes" list that doesn't include collection, an exception is thrown (e.g. {"fields": {"include": ["id"]}}). This is because pgstac doesn't return a collection, and the ItemLinks logic requires it be present.

One option is always to pass collection to pgstac, but then drop it if it's not in fields.include.