Enable HostKeyTest to extract ECDSA and DSA keys

Their certificate-embedded counterparts are enabled as well.

As with RSA, it *is* possible for DSA keys to be of variable length (not
just 1024 bits), so I've added `{'variable_key_len': True}` to the relevant
`HOST_KEY_TYPES` entries, although this key-value pair is otherwise unused.

src/ssh_audit/hostkeytest.py

@@ -55,6 +55,17 @@ class HostKeyTest:
 
         'ssh-ed448':                      {'cert': False, 'variable_key_len': False},
         # '[email protected]': {'cert': True,  'variable_key_len': False},
+
+        'ecdsa-sha2-nistp256': {'cert': False, 'variable_key_len': False},
+        'ecdsa-sha2-nistp384': {'cert': False, 'variable_key_len': False},
+        'ecdsa-sha2-nistp521': {'cert': False, 'variable_key_len': False},
+
+        '[email protected]': {'cert': True, 'variable_key_len': False},
+        '[email protected]': {'cert': True, 'variable_key_len': False},
+        '[email protected]': {'cert': True, 'variable_key_len': False},
+
+        'ssh-dss':                      {'cert': False, 'variable_key_len': True},
+        '[email protected]': {'cert': True,  'variable_key_len': True},
     }
 
     TWO2K_MODULUS_WARNING = '2048-bit modulus only provides 112-bits of symmetric strength'