Enable sending auth metadata event from Rust

Summary: This differential enables Rust code to send auth metadata event to JS.

Test Plan:

1. Apply this diff from @kamil: https://phab.comm.dev/D10327
2. Put a call to send_auth_metadata_to_js in one of Rust functions used in the diff linked above. Unfortunately you have to pass dummy hardcoded strings instead of real data from identity service sue to the issue: https://linear.app/comm/issue/ENG-6067/grpc-on-native-physical-device-not-working
3. Subscribe in JS to auth metadata event (as in previous diffs)
4. Launch the app. Ensure that dummy metadata is logged.

Reviewers: varun, michal, bartek, kamil

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10337