Adding "StateData" property/feature for smooth transition animation between sprites

[Aodaruma]

Is your feature request related to a problem? Please describe.
I would like a feature that enables smooth transition animations for parts and part groups. A similar feature, called "slots," already exists, which allows instant switching of mesh data within objects, commonly used for sprite switching. However, slot indices are discrete values, making smooth transitions or morphing difficult. Hence, I propose adding a new feature.

Describe the solution you'd like

• Introduce a new sprite transition animation feature called "States."
• This feature will allow multiple "states" to be configured for sprites and sprite groups. Users can control these states through a slider in the coa_tools2 UI to animate smooth transitions between sprites.

Describe alternatives you've considered
An alternative would be to modify the existing SlotsData property, but considering compatibility with past projects, it is preferable to retain the slot feature and add it as a new feature.

Additional context
If possible, I would like to link this feature to the functionality for adding a 2D procedural rig.