Create a `passkey-crypto` library

[Progdrasil]

Create a passkey-crypto library for when dealing with more cryptographic algorithms than p256. Currently that is the only algorithm implemented so it is assumed in the methods. But seeing as we need a certain amount of crypto agility and that;

1. NIST has started to no-longer recommend p256 in favor of p384 or p512 and
2. We'll need to start working towards PQ signature algorithms
3. Support for other crypto library backends for other consumers of this library.

We need a way to reliably go from a CoseKey to a private key and back. Also having helper methods to create a public CoseKey directly from the private key would be a nice simplification in the authenticator code.

This could also be the place where a replacement implementation for the coset::CoseKey type and other types we use from coset go so that we can yank it from our tree. That library has been a source of frustration on multiple occasions.

[Progdrasil]

Example cases where coset has been an annoyance:

• panicking on non-empty buffer after serialization. in the case where an Authenticator supports extensions, the attestation format concatenates 2 CBOR maps together (rather than adding a byte to determine a 1 or 2 item array 🙄) where the CoseKey's map is BEFORE the extension map.
• coset::CoseKey takes ownership of it for serialization, requiring unecessary cloning. This is presumably for dropping sensitive key material from memory, which makes the next point even more confusing...
• It doesn't implement Zeroize to TRULY remove data from memory.

[Progdrasil]

Would enable #50