[Suggestion] Allow using external sprites for animations (Robot, Spider, etc)

[Asterveila]

I had already suggested this via twt but figured i'd open it here too
i'd love if this mod allowed for use of external sprites in animation files (such as the Robot and Spider ones), since these are limited to only using the sprites from their icons (so all 4 parts of the robot and spider), however being able to call for external custom sprites would unlock nearly endless possibilities.

Reason i'd love this myself is i had a texture pack that exploited the anim system to add a Sonic Sprite Animations to robot icons in 2.1, unfortunately this cannot be made as good since 2.2 has the robot's idle animation, therefore i'd need another sprite that i don't have any space for, if i could use external sprites i could not only port it but make it even higher quality