Add hybrid fallback URL extension

[harshlal028]

Description

Define a new extension to carry a fallback URL for hybrid transports, allowing the remote device to offer a fallback experience if no passkeys are available on the device.

Requirements

• Fallback URL must be same-site with RP ID domain (this would currently restrict custom schemes, needs discussion)
• Restricted to hybrid transports by WebAuthn Client

Expected behavior by remote device will be defined by FIDO CTAP.