FLINK-18778: Support the SupportsProjectionPushDown interface for Loo…

[atris]

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[wuchong]

Hi @atris , this task doesn't mean to make LookupTableSource extends SupportsProjectionPushDown. But to make LookupTableSource can have the ability of SupportsProjectionPushDown. Currently, only ScanTableSource supports SupportsProjectionPushDown. We need to update the planner and optimizer to support this.

I will close this PR for now.

Besides, @Hailong has taken this issue. I'm not sure whether he has been starting working on this. Would be better to coordinate the work in the JIRA issue.