signer: Add abstract Key class, implement private key uri scheme for Signer

mypy.ini

@@ -2,6 +2,7 @@
 warn_unused_configs = True
 files =
     securesystemslib/util.py,
+    securesystemslib/signer/*.py,
     securesystemslib/storage.py,
     securesystemslib/gpg/constants.py
 

securesystemslib/exceptions.py

@@ -138,3 +138,12 @@ class StorageError(Error):
     backend."""
 
     pass  # pylint: disable=unnecessary-pass
+
+
+class UnverifiedSignatureError(Error):
+    """Signature could not be verified: either signature was incorrect or
+    something failed during process (see VerificationError)"""
+
+
+class VerificationError(UnverifiedSignatureError):
+    """Signature could not be verified because something failed in the process"""

securesystemslib/signer/__init__.py

@@ -0,0 +1,15 @@
+"""
+The Signer API
+
+This module provides extensible interfaces for public keys and signers:
+Some implementations are provided by default but more can be added by users.
+"""
+from securesystemslib.signer._key import KEY_FOR_TYPE_AND_SCHEME, Key, SSlibKey
+from securesystemslib.signer._signature import GPGSignature, Signature
+from securesystemslib.signer._signer import (
+    SIGNER_FOR_URI_SCHEME,
+    GPGSigner,
+    SecretsHandler,
+    Signer,
+    SSlibSigner,
+)