CryptoSigner: support init from PrivateKeyTypes

[lukpueh]

closes #664 (see #664 (comment))

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 dataclasses.

• Otherwise code is mostly moved around:

  • _get_rsa_padding and _get_hash_algorithm helpers are moved unchanged to module scope (these might move out altogether with signer: deduplicate signing scheme dissection  #594)

  • from_securesystemslib_key can now pretty much directly use the constructor, thus no longer requires the from_pem helper

[jku]

Looks good to me, I find this quite readable now.

[jku]

a bit of detail on how to get an instance of CryptoSigner could be useful. I guess something like:

• if you have a privkeyuri for a private key stored in a file, use the generic Signer.from_priv_key_uri()
• if you just want to create a completely new private key, use CryptoSigner.generate_*() (or CryptoSigner() for even more control over key details)
• if you want to use an existing cryptography private key, use CryptoSigner()