This fixes test errors like the following:

```
Exception com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot find a (Map) Key deserializer for type [simple type, class com.yubico.webauthn.data.PublicKeyCredentialDescriptor] [in thread "Test worker"]
```

which also by extension means even if we fix the deserializer resolution,
`JsonIoSpec` will fail with `PublicKeyCredentialDescriptor` as keys because
input/output JSON will not be identical after deserializing from just the
credential ID. Therefore let's change the map key type to `ByteArray`. We can
keep the `PublicKeyCredentialDescriptor` key type in setters, but transform to
`ByteArray` keys during construction of the `PrfAuthenticationInput` value.

The factory functions on PrfRegistrationInput and PrfAuthenticationInput should
be enough.