lib: rename bt_value_array_get_size() -> bt_value_array_get_length()

lib: rename bt_value_array_get_size() -> bt_value_array_get_length()

"Length" is in line with the field terminology
(bt_field_array_get_length()).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Icbb468c0f6d74cba2c5f2f25559981eedec33c5d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1928
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>