Add device class for sensor gas

[frenck]

Context

Gas consumption is measured in volume.

Proposal

Add a device class DEVICE_CLASS_GAS, for measuring gas consumption.

Additionally add the following units:

VOLUME_CM3 = "cm³"
VOLUME_M3 = "m³"

For the frontend it would use icon: mdi:gas-cylinder

This can be used by e.g., the Toon and DSMR integrations.

Consequences

Needs to be implemented.

[MartinHjelmare]

This was solved here:
home-assistant/core#54110