core: dynamically allocate memory for GPIO line request data

core: dynamically allocate memory for GPIO line request data

For bulk requests, the lines will share the struct gpiohandle_request
object. Instead of it being a part of struct gpiod_line, allocate its
memory dynamically and only store the pointer in the line context
structure.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>