[native] add farcaster avatar URL to registration workflow

Summary:
we can fetch the farcaster avatar URL once the user has connected a farcaster account and then pass it through the registration wizard so that it's available on the avatar selection screen

Depends on D13099

Test Plan: tested later in stack by successfully setting avatar from farcaster

Reviewers: ashoat, tomek

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13100