Export the drag'n'drop types

[timroes]

We currently don't export any of the drag'n'drop types that will be passed into the drag'n'drop callback methods a user needs to specify. That causes people to do produce code that either contains any: https://github.com/elastic/kibana/pull/40866/files#diff-5196c51faa9492fb15b9d72ffd692ee3R109 or write some complex helper utilities to get those (https://github.com/elastic/kibana/blob/master/x-pack/legacy/common/eui_draggable/index.d.ts#L17). I think it would make sense to export DropResult and the other parameters that will be passed into the drag'n'drop handlers from EUI directly (so we abstract away from the underlying React DnD library.