[webauthn] Introduce ICloudRecoveryKey

Introduce ICloudRecoveryKey, a class that handles storage and retrieval
of iCloud keychain-backed (Google) keychain recovery factors. This is
modelled after go/folsom-icloud-keychain (internal only).

Bug: 334408282
Change-Id: Ibdcb10116adc0807fa1de6a89126d4667bc08fac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5463185


Reviewed-by: Adam Langley <agl@chromium.org>
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1291982}