Enable JS to subscribe to auth metadata events

Summary: This differential enables JS code on both Android and iOS to subscribe to auth metadata events.

Test Plan:

1. Apply this patch: https://gist.github.com/marcinwasowicz/8ae2a80f5f8dfd0708f9496182d861bb. It is the same as for previous diffs but this time we use Js file that is comitted in this differential.
2. Build Android and iOs apps.
3. In each ensure that every time you type letter in message input you can see console.log with dummy auth metadata.

Reviewers: bartek, varun, kamil, michal

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10336