### What changes were proposed in this pull request?
Handled 'Deduplicate(Keys, Union)' operation in rule 'CombineUnions' to combine adjacent 'Union' operators into a single 'Union' if necessary when using 'Dataset.union.distinct.union.distinct'.
Currently only handle distinct-like 'Deduplicate', where the keys == output, for example:
```
val df1 = Seq((1, 2, 3)).toDF("a", "b", "c")
val df2 = Seq((6, 2, 5)).toDF("a", "b", "c")
val df3 = Seq((2, 4, 3)).toDF("c", "a", "b")
val df4 = Seq((1, 4, 5)).toDF("b", "a", "c")
val unionDF1 = df1.unionByName(df2).dropDuplicates(Seq("b", "a", "c"))
.unionByName(df3).dropDuplicates().unionByName(df4)
.dropDuplicates("a")
```
In this case, **all Union operators will be combined**.
but,
```
val df1 = Seq((1, 2, 3)).toDF("a", "b", "c")
val df2 = Seq((6, 2, 5)).toDF("a", "b", "c")
val df3 = Seq((2, 4, 3)).toDF("c", "a", "b")
val df4 = Seq((1, 4, 5)).toDF("b", "a", "c")
val unionDF = df1.unionByName(df2).dropDuplicates(Seq("a"))
.unionByName(df3).dropDuplicates("c").unionByName(df4)
.dropDuplicates("b")
```
In this case, **all unions will not be combined, because the Deduplicate.keys doesn't equal to Union.output**.
### Why are the changes needed?
When using 'Dataset.union.distinct.union.distinct', the operator is 'Deduplicate(Keys, Union)', but AstBuilder transform sql-style 'Union' to operator 'Distinct(Union)', the rule 'CombineUnions' in Optimizer only handle 'Distinct(Union)' operator but not Deduplicate(Keys, Union).
Please see the detailed description in [SPARK-34283](https://issues.apache.org/jira/browse/SPARK-34283).
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Unit tests.
Closes#31404 from zzcclp/SPARK-34283.
Authored-by: Zhichao Zhang <441586683@qq.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
96bcb4bbe4