[Feature Request] Allow 'Hold Frame' option to be keyframe animated

[rlguy]

The 'Hold Frame' option will hold a selected frame in place regardless of timeline position. The current implementation of this feature was not designed to be keyframe animated. The user interface allows the setting to be animated, and at this moment, is a mistake in the user interface. Since this feature was not designed to be animated, there can be problems with loading the correct frame when jumping around on the timeline, or using the 'Load Last Frame' feature.

This feature request is to redesign the implementaiton of this feature to allow the option to be animated. An example use case for this feature is to enable the hold frame option at a point in the timeline to create a 'freeze frame'. If this feature is not implemented by the next version (1.0.4), ability to insert keyframes on this option should be disabled.

[rlguy]

This feature was implemented by adding an Override Frame feature similar to Blender's mesh cache modifier on Alembic caches. This feature was introduced in FLIP Fluids version 1.0.9, but due to a bug in Blender T71908, the feature ended up not being functional. A workaround for this issue was added to experimental version 9.0.9.12