[FLINK-31118][table] Add ARRAY_UNION function.

[liuyongvs]

• What is the purpose of the change
  This is an implementation of ARRAY_UNION

• Brief change log
  ARRAY_UNION for Table API and SQL

Returns an array of the elements in the union of array1 and array2, without duplicates. 

Syntax:
array_union(array1, array2)

Arguments:
array: An ARRAY to be handled.

Returns:
An ARRAY. If any of the array is null, the function will return null.
Examples:

> SELECT array_union(array(1, 2, 3), array(1, 3, 5));
 [1,2,3,5] 

See also
spark https://spark.apache.org/docs/latest/api/sql/index.html#array_union
presto https://prestodb.io/docs/current/functions/array.html

• Verifying this change
  This change added tests in CollectionFunctionsITCase.

• Does this pull request potentially affect one of the following parts:
  Dependencies (does it add or upgrade a dependency): ( no)
  The public API, i.e., is any changed class annotated with @public(Evolving): (yes )
  The serializers: (no)
  The runtime per-record code paths (performance sensitive): ( no)
  Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: ( no)
  The S3 file system connector: ( no)

• Documentation
  Does this pull request introduce a new feature? (yes)
  If yes, how is the feature documented? (docs)

[flinkbot]

CI report:

• 63b0da3 Azure: SUCCESS

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[liuyongvs]

hi @snuyanzin i submit a new pr and close this #21958.
The new implementation considers the issue of type conversion and refers to the spark implementation https://github.com/apache/spark/blob/50b652e241f7e31b99303359ec53e26a8989a4f0/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/collectionOperations.scala#L4030

[dawidwys]

nit: haystack does not make sense in this context imo. It's a nice joke but imo works only for ARRAY_CONTAINS

[dawidwys]

LGTM