MINOR: obj_type: add OBJ_TYPE_HXLOAD for haload stream objects

MINOR: obj_type: add OBJ_TYPE_HXLOAD for haload stream objects

This patch introduces the OBJ_TYPE_HXLOAD object type to represent
haload stream objects (struct hastream).

It also adds the associated inline helper functions objt_hastream()
and __objt_hastream() to allow safe casting and retrieval of
hastream contexts from a generic object pointer, following the
standard container_of pattern.