Add fan support for KNX climate entities (#126368)

* Add fan mode support to knx climate

* fix linting errors

* remove unneeded None protection from CONF_FAN_PERCENTAGES_MODES

* Update homeassistant/components/knx/climate.py

Co-authored-by: Matthias Alphart <farmio@alphart.net>

* Update homeassistant/components/knx/climate.py

Co-authored-by: Matthias Alphart <farmio@alphart.net>

* Update homeassistant/components/knx/climate.py

Co-authored-by: Matthias Alphart <farmio@alphart.net>

* Update homeassistant/components/knx/schema.py

Co-authored-by: Matthias Alphart <farmio@alphart.net>

* find closest percentage when not in fan modes

* new field for fan speed mode, max steps apply to both step and percentage

* not picking FAN_OFF when the percentage is closest to zero

* add fan zero mode to support auto mode

* use StrEnum for FanZeroMode

* change default to 'percent'

* fix mypy errors

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Co-authored-by: Matthias Alphart <farmio@alphart.net>