### What changes were proposed in this pull request?
In Spark, we have an extension in the MERGE syntax: INSERT/UPDATE *. This is not from ANSI standard or any other mainstream databases, so we need to define the behaviors by our own.
The behavior today is very weird: assume the source table has `n1` columns, target table has `n2` columns. We generate the assignments by taking the first `min(n1, n2)` columns from source & target tables and pairing them by ordinal.
This PR proposes a more reasonable behavior: take all the columns from target table as keys, and find the corresponding columns from source table by name as values.
### Why are the changes needed?
Fix the MEREG INSERT/UPDATE * to be more user-friendly and easy to do schema evolution.
### Does this PR introduce _any_ user-facing change?
Yes, but MERGE is only supported by very few data sources.
### How was this patch tested?
new tests
Closes#32192 from cloud-fan/merge.
Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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