sync: add minimally functional implementation of WEBAUTHN_CREDENTIAL syncing

WEBAUTHN_CREDENTIAL represent passkeys and are already synced by Google
Play Services. This change makes them sync to Desktop clients behind a
new feature flag, that is disabled by default.

WEBAUTHN_CREDENTIAL entities contain encrypted key material (like the
passkey's private key). Recovering the shared secret used to decrypt
these keys is out of scope for this change, so this effectively only
makes available passkey metadata. Also, creating new entities is
unsupported, aside from some basic testing facilities.

Bug: 1223853
Change-Id: Ib47f2ec4f84ebb8c10cc3edbd3b21a7831927c6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4289536
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/main@{#1123925}