Create a CertificateStore backed by LevelDB

This CertificateStore will be used as storage layer for provisioning
client certificates for managed Chrome Profiles. Private keys will be
stored alongside their corresponding certificates, and a consumer
of that Store will eventually expose those identities in mTLS
connections or other attestation mechanisms.

Bug: b:319256086
Change-Id: I9017844e1b30706a242fda3849711cc384235116
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5213410
Reviewed-by: Matt Mueller <mattm@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Hamda Mare <hmare@google.com>
Commit-Queue: Sébastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1250539}