[SPARK-3862][SQL][WIP] MultiWayBroadcastInnerHashJoin.

This patch creates a new physical operator that does multiway inner-equi-broadcast-join, i.e. joining a single large table with a series of small tables (via broadcast).

This is still WIP, but can be picked up by somebody else. Code has not been tested, and there is no optimizer strategy to use this physical operator.

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