[native] Add a new screen to display a QR code as part of a new native login flow
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.
Depends on D8870
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