Credential and key parsing: Validate match

[chrysn]

With the exposed API, a Python program (and, without some deeper knowledge, even a Rust program) that loads a private key and a credential has no way of finding out that the configured credential does not match the provided private key.

Steps I suggest for enhancing the API, at least from a Python PoV:

• Take the private key as a private key object, alternatively (similar to how we take an AutoCredential)
• Give the private key object a method to verify that any given credential matches it. (I'm not sure our crypto API has the tools to do that; worst case, it can be done by generating a key pair and verifying that GXY = GYX).
• By-catch (and that'll be good for algorithm agility anyhow): Add a constructor for the private key object that parses a COSE key, and a serializer. (Nice extra property: once a key is loaded, the Python program can't access it any more outside EDHOC).
• By-catch: Have key pair generation produce a key object.

@geonnave, is there a plan for #277 yet? (That'd influence how the private key object is best done)

Blocks: chrysn/aiocoap#377