[FLINK-25476][table-planner] support CHAR type in function MAX and MIN

[xuyangzhong]

What is the purpose of the change

Now function MAX and MIN does not support the type CHAR and will throw the exception like following:

Caused by: org.apache.flink.table.api.TableException: Max aggregate function does not support type: ''CHAR''. Please re-check the data type. at org.apache.flink.table.planner.plan.utils.AggFunctionFactory.createMaxAggFunction(AggFunctionFactory.scala:395) at org.apache.flink.table.planner.plan.utils.AggFunctionFactory.createAggFunction(AggFunctionFactory.scala:76) at org.apache.flink.table.planner.plan.utils.AggregateUtil$.$anonfun$transformToAggregateInfoList$1(AggregateUtil.scala:444)
This pr aims to add the support in MAX and MIN with both retract and non-retract stream data.

Brief change log

• support CHAR in MAX (Retract&nonRetract).
• support CHAR in MIN (Retract)
• add some test cases

Verifying this change

Some test cases are added to verify this change.

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): no
• 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? no
• If yes, how is the feature documented?

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CI report:

• 9b78b3e Azure: SUCCESS

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Automated Checks

Last check on commit 6436e2a (Mon Jan 17 05:51:52 UTC 2022)

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[RyanSkraba]

This LGTM: the implementation is straightforward, and it seems logical that if VARCHAR is supported for MIN/MAX aggregations, CHAR could be as well.

[lsyldliu]

@xuyangzhong Thanks for your contribution, LGTM overall, I just left a minor comment.