[native] introduce already connected button to farcaster connect

Summary:
This diff introduces the last part to the FarcasterConnect screen: Already connected button. If for some reason the user navigates backwards through the flow + they have already signed in with farcaster, we will show this button so that they do not need to re-sign in with farcaster again to move forward to where they originally were

Depends on D11462

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

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11463