Retrieve manifest image size and architecture from API

[ianballou]

Is your feature request related to a problem? Please describe.
Katello would like to begin showing container image sizes and related architecture information.
We'd also like to show more info about the type of image, but that seems to be covered by #1751

Describe the solution you'd like

• A new field for compressed image size on manifests
• A new field for architecture on manifests

Describe alternatives you've considered
None so far.

Additional context
This is part of an effort for Katello to revamp its container registry UI. Our goal is to show the information that a user would expect from a general container registry UI. If there is other data not yet in Pulp that would be helpful in a UI, lets discuss that here as well.