[web-db] generate and persist encryption key on main thread on Safari

Summary:
Generating/reading, persisting, and exporting to JWK encryption key on Safari.
(context)

Depends on D8120

Test Plan: Check in storage if key is properly persisted.

Reviewers: michal, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8121