[web-db] implement crypto functions

Summary:
Based on SubtleCrypto. Functions we will need for encryption database.

Depends on D6994

Test Plan:
Encryption:

1. Create data ArrayBuffer
2. Encrypt
3. Decrypt
4. Compare

Crypto Key:

1. Check if it's properly generated
2. Verify that can not be extracted using js

Reviewers: michal, tomek, bartek

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6995