Implement CommServicesAuthMetadataEmitter native event emitter on iOS

Summary: This differential implements CommServicesAuthMetadataEmitter for iOS in such a way that it is callable from C++.

Test Plan:

1. Apply this patch: https://gist.github.com/marcinwasowicz/7f04bd592dd4a800adbc96b720e200b7. It creates js code to subscribe for auth metadata events and sends this event from C++ upon each draft update (contents of the event are just dummy strings).
2. Build iOS app.
3. Ensure that each time you write one letter you can see console.log with dummy auth metadata in native yarn dev terminal.

Reviewers: bartek, kamil, michal

Reviewed By: bartek

Subscribers: varun, ashoat, tomek

Differential Revision: https://phab.comm.dev/D10334