Add external humidity sensor support and update documentation

[aes-alienrip]

No description provided.

[p-monteiro]

External Humidity Sensor: Select a humidity sensor entity to use instead of the built-in AC sensor. Choose 'None' to use the built-in sensor. This is useful if you have a more accurate room humidity sensor that you want the AC to use for humidity readings

Keep in mind that the AC device will not actually use this for its operation; this is only the reported value shown in HA

[RobHofmann]

Tested this against a fake Gree device and a real one. Works as advertised: the override applies, it falls back cleanly when the external sensor is unavailable or non-numeric, and the new select entity loads with no errors.

One thing to fix first.

The dropdown lists the integration's own sensor.<device>_room_humidity as a choice, because the filter matches anything with device_class: humidity. Pick it and the integration starts reading its own output, so the humidity value freezes at whatever it was and never updates again until you change the dropdown back. No error, no warning, it just stops moving.

That option is never useful: if the AC has no built-in humidity sensor it is dead, and if it does have one, picking it should mean the same as "None". Simplest fix is to skip this integration's own entities when building the list. The temperature picker has the same gap, but there it only produces a wrong value instead of a frozen one, because external temperature and the outside temperature sensor use separate variables.

Smaller things:

• en.json has the new key, the other 10 translation files do not.
• The temperature picker also matches on unit (°C/°F/K) so helpers and template sensors show up. The humidity one only matches device_class: humidity, so a % template sensor without a device_class will not appear. Intentional?
• This also adds current_humidity to the climate entity, so the thermostat card now shows humidity for everyone, not just users of the new picker. Works fine, just worth noting since it is not in the title.