[SPARK-22018][SQL]Preserve top-level alias metadata when collapsing projects

[tdas]

What changes were proposed in this pull request?

If there are two projects like as follows.

Project [a_with_metadata#27 AS b#26]
+- Project [a#0 AS a_with_metadata#27]
   +- LocalRelation <empty>, [a#0, b#1]

Child Project has an output column with a metadata in it, and the parent Project has an alias that implicitly forwards the metadata. So this metadata is visible for higher operators. Upon applying CollapseProject optimizer rule, the metadata is not preserved.

Project [a#0 AS b#26]
+- LocalRelation <empty>, [a#0, b#1]

This is incorrect, as downstream operators that expect certain metadata (e.g. watermark in structured streaming) to identify certain fields will fail to do so. This PR fixes it by preserving the metadata of top-level aliases.

How was this patch tested?

New unit test

[marmbrus]

LGTM

[SparkQA]

Test build #81801 has finished for PR 19240 at commit b3e41a7.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.