[web-db] generate CryptoKey and persist

Summary:
Generating crypto key for user and persist in indexedDB.

Depends on D6995, D6993

Test Plan:
Run this code:

await webDBWorker.scheduleOnWorker({
  type: workerRequestMessageTypes.GENERATE_CRYPTO_KEY,
});

in check in indexedDB devtools if crypto key is properly persisted

Reviewers: michal, tomek

Reviewed By: michal

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6996