Add CometBroadcastExchangeExec to support broadcasting the result of Comet native operator #81

What is the problem the feature request solves?

We need to support broadcasting the result of Comet native operator. This is another step towards supporting BroadcastHashJoinExec.

Like Spark BroadcastExchangeExec, CometBroadcastExchangeExec is not directly called with doExecute or doExecuteColumnar as other operators. CometBroadcastExchangeExec is used when Spark query planner inserts BroadcastDistribution on top of an operator to broadcast it. The upstream operator of CometBroadcastExchangeExec, e.g., BroadcastHashJoinExec, will call executeBroadcast method of CometBroadcastExchangeExec to execute the query plan and broadcast its results.

Describe the potential solution

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Additional context

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