CredProps::rk should be public

[smessmer]

Is your feature request related to a problem? Please describe.
Because of issues with the webauthn-rs serialization discussed in other Github issues, we're using our own custom serialization logic for passkeys. This means we serialize our own custom types and need access to the internals of the passkey struct so we convert from/to our own custom types. We do have access to most, but CredProps::rk is a field that we don't have access to. It is accessible from a passkey via the Credential::extensions property.

Describe the solution you'd like
Make CredProps::rk public

Describe alternatives you've considered
Ideally, we wouldn't have to do our own custom serialization logic, i.e. webauthn-rs would itself have a space-efficient and backwards compatible serialization layout. But until that's the case, this way works for us.

[Firstyear]

I was thinking we need to address the serialisation side soon. @micolous We should catch up and resolve this once and for all.