Add plumbing for passing credentials to devices (#507)

* Add plumbing for passing credentials as far as discovery

* Pass credentials to Smart devices

* Rename authentication exception

* Fix tests failure due to test_json_output leaving echo as nop

* Fix test_credentials test

* Do not print credentials, fix echo function bug and improve get type parameter

* Add device class constructor test

* Add comment for echo handling and move assignment

kasa/credentials.py

@@ -0,0 +1,12 @@
+"""Credentials class for username / passwords."""
+
+from dataclasses import dataclass, field
+from typing import Optional
+
+
+@dataclass
+class Credentials:
+    """Credentials for authentication."""
+
+    username: Optional[str] = field(default=None, repr=False)
+    password: Optional[str] = field(default=None, repr=False)