Support alternative credentials (2FA, passwordless, webauthn)

[sandhose]

There are a bunch of fun alternative credentials we should explore. That includes:

• 2FA via OTP or WebAuthn
• main credential via WebAuthn only
• passwordless login (send email, click link)

[olanod]

webauthn-rs could be used to implement an experimental WebAuthN support. I was planning in using that to build my own auth server for a matrix based application that should have passwordless single factor authentication. Glad to see the plans for Authentication Services and better that the implementation is in Rust :)

[matrixbot]

For your information, this issue has been copied over to the Element fork of matrix-authentication-service: element-hq/matrix-authentication-service#18