Display metadata in repo results

Created by: lguychard

For background on what repository metadata is, see RFC 729. At a high level, repo metadata will be user-defined, arbitrary key-value pairs associated with repositories, and searchable through predicates.

Because this information will be searchable, displaying what has been searched might be helpful to the user: we might want to display the key/value pairs from repo metadata in repo matches in the UI.

At a high level, the current structure of repository results looks like it may be extensible to fit in this metadata:

/cc @benvenker @lguychard