[Feature request]: Add LED backlight brightness control for Gewiss GWA1201_TWO_WAY_SWITCH

Is your feature request related to a problem? Please describe

The Gewiss GWA1201_TWO_WAY_SWITCH has a status LED that is currently
very bright and cannot be dimmed through Zigbee2MQTT. In the
manufacturer's own Home Gateway app, this LED brightness is
adjustable, but Zigbee2MQTT does not expose this option, so I have
no way to reduce the LED intensity after migrating the device away
from the Gewiss hub.

Describe the solution you'd like

The Gewiss GWA1201 (and likely other ChorusMart devices) supports
adjusting the internal status LED brightness via the manufacturer's
Home Gateway app, but this attribute is not currently exposed in
Zigbee2MQTT.

Current exposes: switch, power_on_behavior, power, voltage, current,
energy, identify.

Would it be possible to add support for reading/writing LED backlight
brightness, similar to what's documented in the Gewiss manual?

Device: GWA1201_TWO_WAY_SWITCH
Z2M definition: https://www.zigbee2mqtt.io/devices/GWA1201_TWO_WAY_SWITCH.html

Describe alternatives you've considered

• Physically covering the LED with tape/diffuser material to reduce
  visible brightness — works but is a workaround, not a real fix,
  and looks unprofessional on a wall switch.
• Manually inspecting the device's Zigbee clusters via the Z2M
  frontend (Dev console) to find an undocumented manufacturer-specific
  attribute for LED brightness, without success so far.
• Staying on the Gewiss Home Gateway hub instead of migrating to
  Zigbee2MQTT, just to keep LED brightness control — not desirable
  since it means giving up local/open-source integration with Home
  Assistant.

Additional context

Gewiss product page:
https://www.gewiss.com/ww/en/products/product.1000000.1008169.GWA1201

Related Gewiss ChorusMart manual (for a similar device, GWA1241)
mentions the status LED and its behavior:
https://manuals.plus/gewiss/chorusmart-connected-axial-2-command-module-manual

LED states documented: fixed red (not configured), fixed blue 50%
brightness (normal operation), flashing blue (identification),
fixed yellow (startup), fixed green (network open), alternating
green/red (factory reset).

This suggests the device already has brightness-level control
internally (the "50%" reference), which may be exposed through a
manufacturer-specific cluster not yet reverse-engineered in the
current Z2M definition (contributed in v2.1.0, PR #8601).

Start with the GWA1201_TWO_WAY_SWITCH definition linked in the issue and review the related contribution in PR #8601. Use the Gewiss manual and the Zigbee2MQTT frontend Dev console to investigate the undocumented manufacturer-specific attribute. Done means exposing readable and writable LED backlight brightness for the device.