Introduce UnitConverter protocol (#78888)

* Introduce ConversionUtility

* Use ConversionUtility in number

* Use ConversionUtility in sensor

* Use ConversionUtility in sensor recorder

* Add normalise to ConversionUtility

* Revert changes to recorder.py

* Reduce size of PR

* Adjust recorder statistics

* Rename variable

* Rename

* Apply suggestion

Co-authored-by: Erik Montnemery <erik@montnemery.com>

* Apply suggestion

Co-authored-by: Erik Montnemery <erik@montnemery.com>

* Apply suggestion

Co-authored-by: Erik Montnemery <erik@montnemery.com>

Co-authored-by: Erik Montnemery <erik@montnemery.com>

homeassistant/helpers/typing.py

@@ -1,7 +1,7 @@
 """Typing Helpers for Home Assistant."""
 from collections.abc import Mapping
 from enum import Enum
-from typing import Any, Optional, Union
+from typing import Any, Optional, Protocol, Union
 
 import homeassistant.core
 
@@ -26,6 +26,16 @@ class UndefinedType(Enum):
 
 UNDEFINED = UndefinedType._singleton  # pylint: disable=protected-access
 
+
+class UnitConverter(Protocol):
+    """Define the format of a conversion utility."""
+
+    VALID_UNITS: tuple[str, ...]
+
+    def convert(self, value: float, from_unit: str, to_unit: str) -> float:
+        """Convert one unit of measurement to another."""
+
+
 # The following types should not used and
 # are not present in the core code base.
 # They are kept in order not to break custom integrations