indicator_leds.c doesn't compile on a split peripheral #3307

indicator_leds.c calls zmk_endpoint_is_connected(), which isn't compiled on split peripherals. We need to do one of two things to allow indicator LEDs to be used on peripherals:

1. Compile a subset of the endpoints code for split peripherals and have the central send its current endpoint state to the peripherals so they can use it.
2. Change indicator_leds.c to ignore the endpoint state on peripherals. It will then not be able to match the same behavior as a central, since it won't know when it should change LED brightness because the keyboard is disconnected.

Option 1 would be preferred, however not all functions can or should be implemented.

The following are pure functions, so they will work as-is:

• zmk_endpoint_instance_eq
• zmk_endpoint_instance_to_str

The following functions could be implemented if current_instance and preferred_transport were mirrored from the central to the peripherals:

• zmk_endpoint_get_preferred_transport
• zmk_endpoint_get_preferred
• zmk_endpoint_get_selected
• zmk_endpoint_is_connected

zmk_endpoint_instance_to_index and ZMK_ENDPOINT_COUNT cannot be accurately implemented on peripherals, as they depend on the local CONFIG_ZMK_USB, CONFIG_ZMK_BLE, and ZMK_BLE_PROFILE_COUNT values, which may differ from the ones on the central.

zmk_endpoint_set_preferred_transport and zmk_endpoint_toggle_preferred_transport could theoretically be implemented on peripherals, but I don't see a reason that a peripheral would need to send a message to the central to directly change its preferred transport.

The following functions don't make sense to implement on a peripheral, since only the central sends HID data:

• zmk_endpoint_send_report
• zmk_endpoint_send_mouse_report
• zmk_endpoint_clear_reports