[native] implement onSuccess callback for ConnectFarcaster registration step

Summary:
This diff implements the onSuccess callback in the ConnectFarcaster screen. This callback gets called back whenever we successfully get the fid from the farcaster webview

Depends on D11461

Test Plan:
Please see the demo video below + tested the entire sign in flow during the hackathon

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11462