Support update method rebase

[mcwarman]

I couldn't find any documentation on this for the API endpoint but after some research into the requests the UI is making itself it is possible to update PRs with a rebase using the following option:

gh api \
  --method PUT \
  -H "Accept: application/vnd.github+json" \
  -H "X-GitHub-Api-Version: 2022-11-28" \
  "/repos/$org/$repo/pulls/$pr/update-branch" \
  -f update_method=rebase

Could this be a configurable option?