expose dropdown properties to group nodes

We need the ability to expose dropdowns to the group inputs, otherwise TD's need to create hundreds of groups if the end user isn't expected to edit the node trees.

To overcome the issue of different dropdown options changing the node's input sockets:

When the socket is connected to the group input, the node shows a new checkbox named ‘expose compatible only’. If ticked, the dropdown on the front end group node only shows options with the same socket names. If unticked the user can tab into the node (similar to a group node), where they’re met with x duplicates of the outer node, one for every dropdown option. So if you have 10 options in the dropdown, you’d see 10 copies of the node, each with a different option selected. The ones that have the same sockets as those already wired to on the outer node will already be wired. The ones that don’t, the user will need to wire up to tell blender what goes where if the user selects one of those options on the outer node.

Getting these exposed to the group inputs and modifier front end will be monumental for TD’s and end users with insufficient knowledge of the inner workings of the modifier’s nodes.