[PasskeyUnlockErrorUI] Add the flag for Passkey Unlock Error UI

This change introduces a new feature flag `PasskeyUnlockErrorUi`.
It enables displaying a UI that encourages the user to unlock their
passkeys on Windows, macOS, Linux, and ChromeOS. The new UI will be
shown to passkey users in case when their access to passkeys is “locked”
and when they have an available user verification mechanism (either a
system UV or a GPM PIN). The proposed passkey unlock UI is similar to
the existing Zuul unlock UI.

Change-Id: I6a04746f3d0b65735420230dbc3397b27606f8ba
Bug: 442809067
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7020072
Reviewed-by: Adem Derinel <derinel@google.com>
Commit-Queue: Ivana Žužić <izuzic@google.com>
Reviewed-by: Yurii Lahodiuk <ylahodiuk@google.com>
Cr-Commit-Position: refs/heads/main@{#1526859}