Realm: Refactor GPUIndirectXferDes by adding a scatter/gather gpu channel

This PR refactors GPUIndirectXferDes in the following way:

• Adds a separate channel GPUScatterGatherChannel for cuda gather/scatter operations which allows to "tweak" heuristics for the path planner ..e.g. frag_overhead. This allows the path planner to choose this channel and therefore enable for example cuda gather operation into a GPU_FB on a remote node. In addition, the new channel allows to override the supports_path which allows to move all the decision making logic (on whether a gpu scatter/gather is possible) into the channel itself.

**PR does not contain any functional changes.