[native] Add JSI bindings for LogInExistingDevice RPC

Summary:
Exposed the LogInExistingDevice to JS - added it to CommRustModule

Depends on D11469

Test Plan:
Called the RPC from JS on native. Repeated test plan from D11466.
Additionally, triggering commCoreModule.setCommServicesAuthMetadata() with the new token is able to refresh user session

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11470