[web-db] implement types for SubtleCrypto module

Summary:
There are no build in flow types for SubtleCrypto or any implementation we can use.

Adding types based on API docs and typescript.

Test Plan: N/A

Reviewers: michal, tomek, bartek, ashoat

Reviewed By: bartek, ashoat

Subscribers: bartek, ashoat, atul

Differential Revision: https://phab.comm.dev/D6994