footprint: Add Hearing Access Hearing Aid BT Audio sample

Add the hearing aid sample to the footprint tracking for the
nrf52840dk_nrf52840 and nrf5340dk_nrf5340_cpuapp.

The sample should represent a simple hearing aid implementation,
and is thus a very good candidate for tracking the footprint
of LE Audio in Zephyr.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>