Implement `retain_checkpoints`

[alexjfisher]

Is your feature request related to a problem? Please describe.
retain_checkpoints was implemented in pulpcore for file repositories in pulp/pulpcore#7437, but hasn't been implemented for rpm repositories.

Describe the solution you'd like
The ability to use this feature with rpm repositories.

[ggainey]

RpmRepository is-a pulpcore Repository - so it inherits retain_checkpoints "for free" - -but pulp-cli doesn't recognize it. Moving this issue to pulp-cli