support virtual collection filter by collection id

[tomkralidis]

Overview

This PR updates query collections by supporting a collection argument. The context is when querying for a virtual collection, a pycsw repository plugin should be able to filter by collection to reduce overhead when querying for items in a virtial collection.

Related Issue / Discussion

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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