[web-db] add methods to import/export key to JWK format

Summary:
JWK is JSON objects representation of private, public, and secret keys (docs).
This allows transferring key between threads on Safari, structure clone algorithm should handle JWK format which is a simple JS object.
(context)

Depends on D8118

Test Plan: Run tests

Reviewers: michal, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8119