[native] Implement thumbhash generation on iOS

Summary:
Implemented the generateThumbHash() function on iOS.
Used code from the thumbhash repository: https://github.com/evanw/thumbhash

Depends on D7778

Test Plan: One easy way is to hook this to expo-image-picker result (e.g. avatar picker) and provide picked uri to the function. It should usually return a 43 char long string (sometimes longer if there is alpha channel).

Reviewers: atul, marcin, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7779