Expose MassData to reduce boilerplate code

[AlmasB]

Discussed in #1432

^{Originally posted by LifeDrainingCoding September 15, 2025}
Is there in FXGL 21 any way to configure Entity mass except adding fixture definitions with density? Btw, MassData is located in package that's not visible. I think it's essential to expect mass property of physical bodies in game engine but i didn't find any except density.