[native] Add base64 decode to CommUtilsModule

Summary:
This adds JSI bindings for D7758 to the native CommUtilsModule.

Depends on D7758.

Test Plan:
Encoded and decoded a Uint8Array into base64, using commCoreModule.base64{Encode,Decode}buffer() and console-log.

Used three different length of Uint8Arrays to test all padding values, e.g.:

new Uint8Array([1,2,3,4,5,6]);  // AQIDBAUG
new Uint8Array([1,2,3,4,5,6,7]); // AQIDBAUGBw==
new Uint8Array([1,2,3,4,5,6,7,8]); // AQIDBAUGBwg=

Reviewers: kamil, marcin

Reviewed By: kamil

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7760