Allow Groups to have clearly defined inputs and outputs

### Feature Idea

Currently grouping nodes seems to be mostly a visual thing. It would be really cool if a group would act like a node itself.

Such a group node could for example have just an input "image", do some complex upscaling inside and just have an output out_image and the rest of the workflow does not need to know the details what's happening in the group.

One would then, for example, connect the inputs of the first nodes to the left side of the group by dragging the connection there, which creates inputs for the group box, and the same for the outputs at the right side.

### Existing Solutions

One can create group nodes instead of groups, but they do not easily show how they are connected inside and are hard(er) to edit.

### Other

_No response_