[native] Create expo-module for Thumbhash generation

Summary:
Thumbhash generation requires raw pixel RGB data, which is not easily available from JS, but we can easily get it in native code. Created an expo module for that purpose.
Basically copy-pasted the aes-crypto module.

Test Plan: generateThumbHash("foo").then(console.log) prints "unimplemented" on iOS and Android

Reviewers: ashoat, atul, marcin

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7778