add free-text search extension

[hrodmn]

Related Issue(s):

• add support for free-text extension (in collection search) #153

Description:
This is a pretty simple addition but it is dependent on the next release of pgstac (the pgstac-side changes were added in stac-utils/pgstac#295) so the new test will fail until we decide how to conditionally activate this feature. Would it make sense to only activate this extension if the pgstac version is >0.9.1?

The q parameter should probably never be used in an item search because it will probably slow things down a lot, but it is very useful for the collection search function.

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.

[vincentsarago]

@hrodmn I would love to first finish #155 which I think is going to be dependent on 0.9.2 (at least on the tests side)