[FLINK-26943][table] Add the built-in function DATE_ADD

[dylanhz]

What is the purpose of the change

Add DATE_ADD supported in SQL & Table API. Examples:

> SELECT date_add('2016-07-30', 1); 
2016-07-31  

Brief change log

FLINK-26943

Verifying this change

This change added tests and can be verified as follows: TimeFunctionsITCase#dateAddTestCases

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:

• 771647e1a323bb10fd855913ba196ed5c60debfa Azure: SUCCESS

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[github-actions[bot]]

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[github-actions[bot]]

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