fuel_gauge: Add fuel_guage_get_props()

The fuel_gauge_get_prop() function prototype declares a function that
retrieves multiple fuel gauge properties at once. The naming suggests it
ought to fetch a singular property at a time. Moreso, some clients may just
want to fetch properties one at a time and may feel uncomfortable using a
prototype for fetching multiple properties when wanting to fetch them one
at a time.

Modify fuel_gauge_get_prop() to fetch a single property and add
fuel_gauge_get_props() to support fetching multiple properties. Modify
existing tests/drivers/samples.

This is part of #61818 work.

Signed-off-by: Aaron Massey <aaronmassey@google.com>