Add DeviceConfig to allow specifying configuration parameters (#569)

* Add DeviceConfig handling

* Update post review

* Further update post latest review

* Update following latest review

* Update docstrings and docs

kasa/tests/test_deviceconfig.py

@@ -0,0 +1,21 @@
+from json import dumps as json_dumps
+from json import loads as json_loads
+
+import httpx
+
+from kasa.credentials import Credentials
+from kasa.deviceconfig import (
+    ConnectionType,
+    DeviceConfig,
+    DeviceFamilyType,
+    EncryptType,
+)
+
+
+def test_serialization():
+    config = DeviceConfig(host="Foo", http_client=httpx.AsyncClient())
+    config_dict = config.to_dict()
+    config_json = json_dumps(config_dict)
+    config2_dict = json_loads(config_json)
+    config2 = DeviceConfig.from_dict(config2_dict)
+    assert config == config2