Make projectile entities able to use scale attributes

The new scale attribute is incredibly fun to work with, but it's a shame that projectile entities cannot be changed using it. This is particularly noticable when players or skeletons are scaled up or down a lot, since the arrows do not properly change size to reflect who shot it. Allowing projectile entities to use the scale attribute would allow players to do cool things with them in datapacks, such as having arrows visually represent how much damage they would do by changing size.