Support 'Alternate Content Sources' within pulp3
Alternate content sources were useful in pulp2 for the following scenario:
A master pulp server living on premise
A 'mirror' pulp server living in the cloud
An existing yum mirror existing in the cloud alongside the pulp mirror (aka RHUI)
Being able to sync the mirror from the master pulp server, but pull the actual RPMs from the existing yum mirror allows to cut down on bandwidth costs within that cloud instance.
It still needs to pull the metadata for the mirror from the master pulp server, as there may be 'curated' (aka some subset) of content there.