Add support for passkeys (WebAuthn as primary authentication)

[Saklad5]

Feature Description

Passwords are a pain to deal with, and best avoided. Multi-device FIDO credentials, also known as passkeys, are intended to supplant passwords and multi-factor authentication entirely by using public keys as the sole method of authentication.

Because they use the WebAuthn specification, minimal changes are needed for an application like Gitea to support them: simply allow accounts to be set up with a WebAuthn key instead of a password, and adjust the login flow accordingly.

I'd also like the ability to disable password support entirely. While passkey support is still being rolled out across the major operating systems and browsers, my own systems all support it right now.