[v4] skip_types should be removed from the sync-time options, added to the remote options

[dralley]

Is your feature request related to a problem? Please describe.

skip_types being a sync-time option is awkward because A) it's one of very few such options and B) it's the sort of thing that for most people would remain static over the lifetime of the repo, and changed infrequently if ever.

Describe the solution you'd like

skip_types should be a remote option as opposed to a sync-time option

[ggainey]

We can start by adding to v3 Remote, and teach sync to look there in addition to on-sync. Should this be two issues?