drivers: adc: Introduce the adc_sequence_options::user_data field

Introduce the field that can be used to associate a given sampling
sequence with any other data needed in the sampling-done callback
function.
Adjust one ADC API test case that uses a sequence callback so that
it checks if this introduced field is propagated as expected.

Also clarify in the description of the `sequence` parameter of the
callback function that this parameter is not supposed to be supplied
to the CONTAINER_OF() macro, to avoid any further confusion in that
regard.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>