add support for free-text extension (in collection search)

[hrodmn]

This is an important addition to the collection-search functionality. It is already implemented in pgstac (unreleased) but it would be great to add this to the list of features to add when we add support for pgstac>=0.9.

The q parameter for free-text search works fine for collections because of the limited number of records but could be very slow for item-level search since pgstac builds the text indexes on-the-fly. I suppose we can enable q in both item- and collection search functions but it is not a good idea to use it for items!