[lib] Rename Neynar types

Summary:
We are using FarcasterUser as a type name elsewhere in the codebase, and I'm about to introduce a query that will return users from Neynar but without the ViewerContext.

Figured these names would be less ambiguous, but open to other ideas.

Depends on D11874

Test Plan: Flow

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11875