Return content_guard PRN from distribution

[ianballou]

Is your feature request related to a problem? Please describe.
In Katello, we save content guard hrefs from data returned from the distribution API. To switch to using only PRNs - we'd ideally be able to get the PRN from the same source.

Describe the solution you'd like
Have a new content_guard_prn data point returned with distributions.

Describe alternatives you've considered
Otherwise we can look up the content guard and save the PRN.

[ggainey]

One clarification here - you can't retrieve an entity using just the PRN. PRNs are useful for things like prn__in in list-calls, for example - but to do a GET on the entity itself, we still require HREFs, because you need the routing to know what you're asking for and what you're getting back.