webauthn: store the public key of the PIN virtual member

In order to change a GPM PIN, the EnclaveManager will need to know the
public key of the existing member because the RPC requires that
information.

This change updates the EnclaveManager to store that public key when a
GPM PIN is already set in a security domain.

Note: this invalidates enclave registrations by adding a new invariant
to the state file. The code will consider most state files to be corrupt
and will replace them. Developers will have to re-enroll.

Change-Id: Id0af0e1305cf3a4b15cf24f181261b4c336298b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5407361


Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282191}