Cross-provider model catalog (`codewhale models --all`)

[Hmbown]

Shape. Iterate every configured provider, call list_models() on each, merge results with the static ModelRegistry, group by ModelFamily. New flag --all on the models subcommand. Adjacent to #1891.

Depends on: #2075, #2081.

Slate item 15/18. Tracking doc: .private/issue-slate.md (maintainer-only).