Allow switch nodes to switch between field and single value outputs

Currently, if one input to a switch node is a field, the output will always be a field, whether the corresponding input is or not.

Initially brought up as a bug: projects.blender.org/blender/blender/issues/137304

It has been deemed more feature request territory to change this behaviour, so I've taken it here.

I propose making it so a switch node can distinguish between a field and a single value in geometry nodes for the output.

It is especially useful for the use of baked simulations and saving storage space. If one input for the simulation is not selected, you can use a switch node to use a single value of 0 for baking each frame, instead of 0 as a value on every geometry element.

Image is of the issue.