Add optional value_fn to EntityDescriptions

eg. to be able to map a API response value to a different value (status_code -> message)

homeassistant/components/fronius/sensor.py

@@ -1,6 +1,7 @@
 """Support for Fronius devices."""
 from __future__ import annotations
 
+from collections.abc import Callable
 from dataclasses import dataclass
 from typing import TYPE_CHECKING, Any, Final
 
@@ -85,6 +86,7 @@ class FroniusSensorEntityDescription(SensorEntityDescription):
 
     default_value: StateType | None = None
     response_key: str | None = None
+    value_fn: Callable[[StateType], StateType] | None = None
 
 
 INVERTER_ENTITY_DESCRIPTIONS: list[FroniusSensorEntityDescription] = [
@@ -630,6 +632,8 @@ class _FroniusSensorEntity(CoordinatorEntity["FroniusCoordinatorBase"], SensorEn
         new_value = self.coordinator.data[self.solar_net_id][self.response_key]["value"]
         if new_value is None:
             return self.entity_description.default_value
+        if self.entity_description.value_fn is not None:
+            return self.entity_description.value_fn(new_value)
         if isinstance(new_value, float):
             return round(new_value, 4)
         return new_value