Enable refreshable tokens on the admin registration endpoint

[cvwright]

This PR adds support for refresh tokens on the admin API registration endpoint /_synapse/admin/v1/register.

See #16641 for more details.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[cvwright]

I'm not sure what's going on with the failing tests. The sytest tests give me run-tests PASSED on my dev machine when run according to the instructions at https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#run-the-integration-tests-sytest

[clokep]

Reran them, the flake should be fixed now?

[erikjohnston]

Thanks!