STAC API: implement queryable collections

[tomkralidis]

Overview

Adds queryable support to /collections per https://github.com/stac-api-extensions/collection-search

Note that this PR removes metadata:main as a STAC API collection, the idea being any STAC collections need to be transacted into pycsw in order to appear on /collections responses.

Related Issue / Discussion

None

Additional Information

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Contributions and Licensing

(as per https://github.com/geopython/pycsw/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pycsw. I confirm that my contributions to pycsw will be compatible with the pycsw license guidelines at the time of contribution.
• I have already previously agreed to the pycsw Contributions and Licensing Guidelines