Previously CryptoSigner instances could only be created via
`from_priv_key_uri` factory method, because:
- CryptoSigner was abstract
- Subclass implmentations were protected

This change makes the CryptoSigner constructor usable directly, so that
signers can be created directly from pyca/cryptography private key
objects.

If a public key is not passed along, it is created from the private key,
with default keyid and signing scheme for key type.

*Details*

Internally, this now (1) uses composition instead of inheritance, and (2)
eliminates one level of abstraction:

(1) CryptoSigner is no longer an abstract base class for protected
rsa, ecdsa and ed25519 "signer" subclasses. It only made creation more
difficult, without advantage. Instead CryptoSigner holds references to
the "signers"

(2) There is no more abstraction over individual pyca/cryptography rsa,
ecdsa and ed25519 private key types. cryptography provides a common
"PrivateKeyTypes" interface with sign method, which we can use directly.
The different additional sign args for the different key types are
managed in new internal data classes.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>