getAssertion: Handle multiple matching credentials in allowList

[robin-nitrokey]

Currently, we only select a single credential if an allow list is set. It should also be possible to match multiple credentials.

https://github.com/Nitrokey/fido-authenticator/blob/bb80cba2c28e8a6b19c031ee7d52aadf8c37c801/src/ctap2.rs#L1191

Transferred from Nitrokey#120.