Add support for Bitwarden-style passkey login (WebAuthn / PRF)

[heapframe]

Bitwarden has introduced vault decryption using PRF-enabled WebAuthn credentials:
https://bitwarden.com/help/login-with-passkeys/

Request

It would be valuable for Vaultwarden to support this as it would add more consistency between Bitwarden and Vaultwarden

Context

Passkeys are becoming the default authentication model across major platforms. Supporting them in Vaultwarden would:

• Reduce reliance on master password entry
• Keep feature parity with upstream Bitwarden
• PRF is https://bitwarden.com/blog/prf-webauthn-and-its-role-in-passkeys/

[pamperer562580892423]

#4236