Support for Additive Animations?

**Is your feature request related to a problem? Please describe.**
The feature will be new to the `filament-gltfio`. However, it will be helpful for multiple animations as they can be applied to a common animation without messing up the overall animation.

**Describe the solution you'd like**
The implementation from three.js, which involves marking them as additive animations, is ideal for me. However, I have no idea how feasible it is.

**Describe alternatives you've considered**
Another option is to introduce a function that is similar to `applyAnimation()`, which performs the same task but considers additive animations.

**OS and backend**
This feature is open to any OS and will be a feature of the `filament-gltfio` library.