As a REST api user I would like to know which distributions are included in a publication

[dkliban]

Is your feature request related to a problem? Please describe.
I am working on pulp-replica. This plugin discovers distributions at an upstream Pulp and then syncs that content. In order to configure the remote to consume from the AptDistributions, I need to know which distributions are available there. I am able to use the REST API to look up the publication that is backing the distribution, so it would be helpful to have the publication specify what distributions are included.

Describe the solution you'd like

When make a GET request to the AptPublication or VerbatimPublication end point, I would like the response to include which distributions are available in the publication. Ideally, this would be in the same format as the remote configuration of distributions.

The publication creation task should populate this information when creating a publication.