Use enum to represent RegulatoryExtensionVatiant.

This change permits us to capture the extension type at a more
appropriate stage, and simplifies management overall.

Bug: b/358683018
Change-Id: Ifbb815adbf623d697fd29d86bc884e57926f8d09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5804905


Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Tomasz Wiszkowski <ender@google.com>
Cr-Commit-Position: refs/heads/main@{#1345051}