add package.metadata to help enrich melange yamls with metadata tha…

[rawlingsj]

…t can be a struct or a list of pointers

This metadata can be useful to add context to melange yaml files or provide links to related data in the form of a URI for example,

i.e.

package:
  name: foo-1
  version: 0.0.1
  epoch: 0
  metadata:
    - git://github.com/wolfi-dev/versions/foo.json