getNextAssertion: Set numberOfCredentials

[robin-nitrokey]

Currently we always omit numberOfCredentials in the getNextAssertion response even if there are more applicable credentials.

https://github.com/Nitrokey/fido-authenticator/blob/bb80cba2c28e8a6b19c031ee7d52aadf8c37c801/src/ctap2.rs#L144

Transferred from Nitrokey#119.