[native] introduce navigational logic for TagUnfollowedFarcasterChannel screen

Summary: One of our user experiences for tagging a farcaster channel to a community is that we want to allow the user to amnually type the farcaster channel name into a text field to tag the channel if they can't find the channel in the dropdown. This diff introduces all the navigational + boilerplate screen we will need to introduce this user experience

Test Plan: flow + confirmed I was able to navigate to the boilerplate screen

Reviewers: inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11855