Support GPG and SSH keys

[lukpueh]

securesystemslib provides an API to import and export public and private keys in PEM (RSA, ECDSA, also cf. #54) or in a proprietary format (Ed25519).

It would be convenient to extend the API so that users can load, e.g. their existing GPG or SSH keys. Note that securesystemslib does not necessarily have to provide an API to generate those keys. As @aaaaalbert has mentioned, user might even prefer to use their own well-known toolchain.

[lukpueh]

Re SSH:
Our main (#56) crypto backend pyca-cryptography supports loading public rsa and ecdsa ssh keys (ed25519 is not supported), so it should be easy to add wrappers to securesystemslib.
As for private keys, we are already able to handle private rsa and ecdsa ssh keys stored in PEM format. For some reason ed25519 does not seem to work:

>>> securesystemslib.interface.import_ed25519_privatekey_from_file(".ssh/id_ed25519")

Traceback (most recent call last):
  File "<input>", line 1, in <module>
    k = intf.import_ed25519_privatekey_from_file(".ssh/id_ed25519")
  File "/Users/lukp/code/securesystemslib/securesystemslib/interface.py", line 549, in import_ed25519_privatekey_from_file
    key_object = securesystemslib.keys.decrypt_key(encrypted_key.decode('utf-8'), password)
  File "/Users/lukp/code/securesystemslib/securesystemslib/keys.py", line 1689, in decrypt_key
    securesystemslib.pyca_crypto_keys.decrypt_key(encrypted_key, passphrase)
  File "/Users/lukp/code/securesystemslib/securesystemslib/pyca_crypto_keys.py", line 843, in decrypt_key
    json_data = _decrypt(encrypted_key, password)
  File "/Users/lukp/code/securesystemslib/securesystemslib/pyca_crypto_keys.py", line 986, in _decrypt
    raise securesystemslib.exceptions.CryptoError('Invalid encrypted file.')
CryptoError: Invalid encrypted file.

Update:
ssh-keygen's man page state's Ed25519 keys always use the new [OpenSSH rather than the PEM] private key format.

[lukpueh]

Re GPG:

We have implemented GPG support in in-toto.gpg. We should consider moving the implementation to securesystemslib at some point.