[native] Make Farcaster login optional

Summary:
Apple won't let us launch with Farcaster login required as it's against their App Store Guidelines.

Depends on D13308

Test Plan: Confirmed that the "skip" button shows up on release builds

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13309