[FLINK-24547][Table SQL / Planner]A function named row_kind() can get…

[shidayang]

Add a function can get row_kind, the function return "+I","-U","+U","-D"
you can use this function as "select row_kind() as op from xxx".

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Last check on commit 3bc3287 (Thu Oct 14 12:29:47 UTC 2021)

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[Airblader]

There are several things missing from this PR:

1. Expose the function in Table API (Java + Scala) + JavaDocs / ScalaDocs
2. Expose the function in PyFlink + its docs
3. Document this in the list of built-in functions.
4. Add tests for SQL and Table API.

I only reviewed this preliminarily, I don't know whether this function will be accepted or not.

[Airblader]

This comment makes no sense. Copy paste?

[Airblader]

Please import ROW_KIND to shorten this.

[Airblader]

There's a constant for this class name

[shidayang]

@Airblader Thanks for your review, I made some changes

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