[native] Add a new screen to display a QR code as part of a new native login flow

Summary:
This is the native counterpart of D8821. I created a new navigator because this sign in via QR code flow will inevitably have multiple 'steps' in it (like choosing a device name once the QR code is scanned), so I decided to have it done now rather than later.

One important note: The designs are not finalized, but I've spoken with @ted and I'll do another pass on the designs in a later diff once they're ready (this is tracked in ENG-4701).

Addresses ENG-4515

Depends on D8870

Test Plan:
Click on the button and expect the new screen to show

Reviewers: atul, ginsu, tomek, varun

Reviewed By: ginsu

Subscribers: ashoat, ted

Differential Revision: https://phab.comm.dev/D8912