Completed execution path + failing test

[robtandy]

Ported over the concepts from the previous version into this structure. The tests/distributed_execution.rs is now a failing test due to a proto serialization bug around the custom codec.

I don't have time to further look into it today. I would like this PR merged though if possible, and the fix for the failing test as a subsequent PR. This way it unblocks adding examples, tests, and fixes or reworks.

Brief summary of changes, docs and readme to follow in subsequent smaller PRs

• ExecutionStage now has an assign method which will recursively add worker urls to all tasks in the tree. It is currently done for the entire tree when ExecutionStage::execute() is called, but it can be improved by lazily doing it before sending out a child stage to workers.
• The DoGet proto consists of an ExecutionStageProto and the partition to execute.
• PartitionIsolatorExec node was ported over to allow for using RepartitionExecs unchanged, only showing them their allowed partitions
• CombinedRecordBatchStream ported over for scatter gather from child stages

Challenges:

• I could not get the ChannelManager to work with the FlightClient. The high level flight client is a lot easier to use and it seems to want to take tonic Channels. Happy to have this fixed or refactored.
• I believe the codec work is structured well, but it is not working. I did not find the bug, which is halting distributed execution.