PoC: Support for Sign in with QR with MSC4388

[hughns]

This is a implementation of the 2025 version of MSC4108 to support the MSC.

Features:

• it supports generating a QR
• it can be used on a new client that is not signed in, or to link a new client using an existing client
• it only supports generating a QR

Some things that are lacking and won't be done:

• sensible API shape to support migration from 2024 version of MSC4108
• tests
• logging isn't sensible

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).