[RFC/DRAFT/WIP] ZBT-2 coordinator LED support

[TheJulianJES]

DRAFT. Just playing around for now.

Proposed change

This adds experimental support for directly controlling the ZBT-2 coordinator LED using firmware extensions.
The changes work (with the linked bellows PR), but this is very experimental and rough. Nothing that should be merged.

Notes:

• This PR directly accesses bellows internals for the firmware extensions(!)
  • We could either making this more generic using zigpy as an intermediate layer, similar to how "network country code" firmware extensions are handled?
  • Or, we could have bellows expose a "more public API" for directly using firmware extensions, like this.
• When we reset ZBT-2 firmware whilst connecting, the LED color is turned off, as the ZBT-2 does not persist LED state.
  • So, ZHA now creates a task to restore the last LED state in on_add. That way, you'll only see the ZBT-2 LED turn off for a few seconds when ZHA is reloaded (or HA restarted).
  • It might be worth looking into making this persistent within ZBT-2 firmware, though also consider allowing for ways to turn it off, e.g. when switching to Z2M – at least holding down the reset button should also clear the LED state.
• When the light is turned on for the first time by the user, we send a default DEFAULT_COORDINATOR_LED_XY_COLOR color.
  • Currently, this should match the LED_COLOR_WHITE_DIM color from firmware (RGB8(75, 75, 75))
  • This shows up as "white" with 29% brightness in HA when first turning on the light.
  • The router firmware uses another blue color when searching for a network. Should we also use that here?
• There is currently no command for properly turning the light off and handing control back to the firmware.
  • led_manager_clear_pattern(LED_PRIORITY_MANUAL) is not supported via firmware extensions, so ZHA turns the light "black" to turn it off.
  • This might be something to expose in firmware and bellows, though I don't think it realistically matters with current coordinator firmware behavior. E.g. if we wipe network settings, we reset firmware anyway, so it should start blinking again(?)
• It is somewhat confusing that the coordinator device page in HA also shows ZHA light groups.
  • As a config entity, the ZBT-2 LED is somewhat differentiated, but this is still something we should improved:
  • The following PR would change the behavior for Zigbee groups in HA to use a dedicated device/service: Create device registry entries for ZHA group entities home-assistant/core#162307

Additional information

Requires bellows PR:

• Add XNCP coordinator LED support bellows#710