Implement scaling for display entities

[DrZoddiak]

Implements #setScale for Display Entities since they allow for scaling in 3 directions.

[avaruus1]

How does this differ from setting the scale through the Transform or is it just a different way of doing the same thing?

[DrZoddiak]

How does this differ from setting the scale through the Transform or is it just a different way of doing the same thing?

Effectively it's the same thing. Interpolation keys still effect the entity. This PR simply implements a method that already exists but has no implementation.

https://github.com/SpongePowered/Sponge/blob/994fb277df1885974c03289f33563a617c2d355b/src/mixins/java/org/spongepowered/common/mixin/api/minecraft/world/entity/EntityMixin_API.java#L147-L155