`buf beta registry repository list` is slow for large page sizes

[WillAbides]

buf beta registry repository list makes a separate API call to get the owner name of each repository it calls. This results in buf beta registry repository list buf.build --page-size 250 taking 15s while time buf beta registry repository list buf.build --page-size 10 takes only 1s.

[bufdev]

This would be pretty trivial if we added UserService.GetUsers and OrganizationService.GetOrganizations, that's all we really need to do.