QR code for adding a new owned device

This is the receiver part of https://github.com/vector-im/element-web/issues/16240 but preferably the mobile version would allow generating sharing QRs as well.

## Problem
Adding a new (mobile) device now require some fiddling: user have to re-enter homeserver, account name, and password at least, gets a question about cross-signing and possibly even more questions which are proper in the general case but completely unnecessary when we do know that this is an additional device for an already established account.

## Describe the solution you'd like
A QR code could contain all the required information (hs, account_name, a flag for adding_new_device_login), so the user only have to provide a password [or other verification scheme].
Preferably it would work at in-app login (element-mobile versions) instead of giving hs/credentials.

(It may try to open the app as well from the generic QR reader.)