[FLINK-26945][table] Add built-in DATE_SUB function.

[liuyongvs]

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

• Brief change log DATE_SUB for Table API and SQL

  Syntax:
   date_sub(startDate, days) 
  
  > SELECT date_sub(DATE '2019-01-01', 1);
  2018-12-31
  

  See also Spark Hive

• 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:

• a554381cd3cfd6999950ee5b2a5fe470338762f0 Azure: SUCCESS

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[liuyongvs]

hi @twalthr @wuchong do you have time to help review it ?

[liuyongvs]

@flinkbot run azure