Add smart grid features to SmartSavingMode refrigerator enum

My refrigerator (LFXS28566D) supports the three added options as part of it's "smart grid" functionality.  None of the initial three options were in my fridge, so I figure they are mutually exclusive modes (although OFF and SMART_GRID_OFF are similar, as is CUSTOM and SMART_GRID_CUSTOM).

As best I can tell, SMART_GRID_OFF is for when the smart grid feature is disabled overall, SMART_GRID_DEMAND_RESPONSE is something certain power companies can do to tell the fridge not to defrost when the power grid is stressed (I cannot test as I am ineligible), and SMART_GRID_CUSTOM is used whenever the button on the fridge is pushed or "Seasonal Energy Saver" is enabled in the app.

For reference, here is an excerpt from my model's JSON dump:
```json5
{
...
"SmartSavingMode": {
    "type": "Enum",
    "default": "0",
    "option": {
        "0": "@CP_OFF_EN_W",
        "2": "@RE_TERM_DELAY_DEFROST_CAPABILITY_W",
        "3": "@RE_TERM_DEMAND_RESPONSE_FUNCTIONALITY_W"
    }
}
...
}
```

wideq/refrigerator.py

@@ -28,6 +28,9 @@ class SmartSavingMode(enum.Enum):
     OFF = "@CP_TERM_USE_NOT_W"
     NIGHT = "@RE_SMARTSAVING_MODE_NIGHT_W"
     CUSTOM = "@RE_SMARTSAVING_MODE_CUSTOM_W"
+    SMART_GRID_OFF = "@CP_OFF_EN_W"
+    SMART_GRID_DEMAND_RESPONSE = "@RE_TERM_DEMAND_RESPONSE_FUNCTIONALITY_W"
+    SMART_GRID_CUSTOM = "@RE_TERM_DELAY_DEFROST_CAPABILITY_W"
     EMPTY = ""