PRF + HMAC-Secret Extension support

[JamesFieldist]

https://www.linkedin.com/pulse/passkeys-ios-18-automatic-passkey-creation-upgrades-vincent-delitz-jgzzf

One of the major powers of pass keys is that it gives you a device secure (and often portable across devices with a password manager) private key that can be used to create hashes, and encrypt data.

It would be nice if there was a way to use this library to get access to the PRF and +HMAC-secret extensions which are supported on web, ios, macos and android.

Even if the library didn't directly support it, being able to get the right stuff that a package like: https://pub.dev/packages/signer could use would be super helpful.