OARec: add distributed search functionality

[tomkralidis]

Overview

Adds initial support for distributed search by default for installations with server.federatedcatalogues configured:

[server]
...
federatedcatalogues=https://demo.pycsw.org/cite/collections/metadata:main,https://demo.pygeoapi.io/master/collections/dutch-metadata

Note the approach is implementation specific while this is being discussed in the OGC API - Records SWG.

Related Issue / Discussion

None

Additional Information

None

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

[kalxas]

We need to resolve the conflicts with master branch