webauthn: support dispatching to different platform authenticators.

If there is more than one platform authenticator then we need to be able
to trigger them by type when, for example, the user selects a credential
from conditional UI.

This change add infrastructure for that, but should be a no-op. It
should be a no-op even with the added feature flag enabled, but it takes
a little bit of consideration to convince oneself of that. So the
feature flag is used to make it clearly safe as-is.

Bug: 1439987
Change-Id: I4bcbe4d52f6216d207b5365569ced6816a43d910
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4528616


Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147380}