[FLINK-28929][table] Add built-in datediff function.

[liuyongvs]

What is the purpose of the change

Add built-in datediff function

Brief change log

• Syntax:
  • DATEDIFF(expr1,expr2)
• Returns:
  • returns expr1 − expr2 expressed as a value in days from one date to the other. expr1 and expr2 are date or date-and-time expressions. Only the date parts of the values are used in the calculation.
  • This function returns NULL if expr1 or expr2 is NULL.

See more:

• mysql: https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_datediff

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? (yes)
• If yes, how is the feature documented? (docs)

[flinkbot]

CI report:

• e23772a Azure: SUCCESS

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[liuyongvs]

hi @twalthr do you have time to review it? thanks very much!

[liuyongvs]

hi @snuyanzin could you help review it?