power: supply: core: add UAPI to discover currently used extensions

power: supply: core: add UAPI to discover currently used extensions

Userspace wants to now about the used power supply extensions,
for example to handle a device extended by a certain extension
differently or to discover information about the extending device.

Add a sysfs directory to the power supply device.
This directory contains links which are named after the used extension
and point to the device implementing that extension.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20241211-power-supply-extensions-v6-4-9d9dc3f3d387@weissschuh.net
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>