Add custom data pointer to context field

Add general purpose data pointer to struct lttng_ctx_field to keep get_size()
coherent with the record() function for variable length field like sequence.

To access this data, pass lttng_ctx_field to get_size_arg() function. This
function is called if get_size() is null.

Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>