key: consider adding RSAKey, ECDSAKey and ED25519Key classes

[lukpueh]

SSlibKey covers rsa, ecdsa and ed25519 keys in the same class, and implements verification for each key type in a simple if/elif switch (see #585). For the sake of key signature verification alone this seems reasonable.

But now #590 adds separate signers for each of these key types, which each have field for the corresponding public key object, it might make sense to consider separate public key classes. Then we could e.g. validate if a signing scheme is supported for a public key only once in the public key constructor, and in the signer only validate if the passed public key is correct.

What speaks against this is that a signer of a given key type might not support all schemes that a public key supports. This is e.g. the case for ECDSASigner.

[lukpueh]

But now #590 adds separate signers for each of these key types

This is no longer the case. Let's close this issue.