Implementation of #127106.
This is just a visual representation of the field/single/grid
status of sockets to make the workflow more intuitive. With
a visual representation for volume grid sockets, volume features
should be unblocked for further development. The structure type
will also be used to distinguish list sockets in the interface.
Group input nodes now have a "Structure Type" option instead of
the existing "Single Value Only". Usually the auto option should be
enough, but in some cases where the inferencing cannot (yet) make
a clear determination, it can be helpful to choose a specific type.
The new visualization and the group input structure type option
are hidden behind a new experimental option for now.
Pull Request: https://projects.blender.org/blender/blender/pulls/134811
b80f1f5322