Bug 41741: Refactor the domain isolator and new circuit

Merge Info

Related Issues

• #41741 (closed)

Backport Timeline

• Immediate - patchsets for critical bug fixes or other major blocker (e.g. fixes for a 0-day exploit) OR patchsets with trivial changes which do not need testing (e.g. fixes for typos or fixes easily verified in a local developer build)
• Next Minor Stable Release - patchset that needs to be verified in nightly before backport
• Eventually - patchset that needs to be verified in alpha before backport
• No Backport - patchset for the next major stable

Upstream Merging

• Merge to base-browser - typically for !fixups to patches in the base-browser branch, though sometimes new patches as well
  • NOTE: if your changeset includes patches to both base-browser and tor-browser please please make separate merge requests for each part

Issue Tracking

• Link resolved issues with appropriate Release Prep issue for changelog generation

Change Description

First part of the Torbutton refactor!

Let's move the domain isolation + new circuit out of Torbutton!

I think it's a little bit too late for 12.5, so I've created the 13.0 alpha branch already, as any improvement over the current situation of Torbutton will help!

It also includes some changes for the TorStrings commit (move where we add the DTDs we need on macOS) because this branch started as a branch for #40938 (closed).

I can move them to another MR, if the reviewer prefers so.