[native] Add a temporary sign in via QR code button on native

Summary:
While we're still using the traditional form to log in on native, I'm hiding the QR code login flow behind a new login button (similar to what's done with the register (new) button). This is only
shown in development. The next diff will implement the screen that appears when clicking it.

I followed an approach similar to D7495

Addresses ENG-4649

Depends on D8836

Test Plan:
Confirmed that the button appears only in development (only touched the sign in buttons, left register buttons as-is)

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Reviewers: atul, ginsu, tomek, varun

Reviewed By: ginsu

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8870