[native] Add C++ function to decode base64

Summary:
The thumbhash bytes will be stored as base64 string so we need a function to decode it.

This code, similarly to the encode function, is based on this article, with slight modifications.

Test Plan: This will be tested together with the next diff.

Reviewers: kamil, inka, marcin

Reviewed By: kamil

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7758