Vex and Allay should be able to wear player head or mob skull with commands

Until 1.19.2, Vex were able to wear player head and mob skull, but it was remove with the new vex model.

Since it seem to be intended, I think it's a mistake to do that, since it was pretty useful for map maker to create a flying custom mob just by, for example, add a custom player head to a vex and make him invisible.

The new vex model should be able to wear player head and mob skull too, and since vex and allay are similar, allay should be able to wear them too.