[webauthn] Initial add of PublicKeyCredential.getClientCapabilities()

This CL adds the necessary IDL changes and plumbing for implementing
PublicKeyCredential.getClientCapabilities(), and a Blink runtime feature
to guard this method.
The actual authenticator implementations of the method will be added in
follow-up changes.

Bug: 360327828
Change-Id: I8804eac4c81416d5f83786dfcc61e71f683c4d37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5804809
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Commit-Queue: Andrii Natiahlyi <natiahlyi@google.com>
Cr-Commit-Position: refs/heads/main@{#1346237}