[FLINK-26940][table] Add the built-in function SUBSTRING_INDEX

[dylanhz]

What is the purpose of the change

Add SUBSTRING_INDEX supported in SQL & Table API. Examples:

> SELECT substring_index('www.apache.org', '.', 2);
 www.apache 

Brief change log

FLINK-26940

Verifying this change

This change added tests and can be verified as follows: StringFunctionsITCase#substringIndexTestCases

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:

• 944b606b33aa4c89d18d68a330100602ca046aa8 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[github-actions[bot]]

This PR is being marked as stale since it has not had any activity in the last 90 days. If you would like to keep this PR alive, please leave a comment asking for a review. If the PR has merge conflicts, update it with the latest from the base branch.

If you are having difficulty finding a reviewer, please reach out to the community, contact details can be found here: https://flink.apache.org/what-is-flink/community/

If this PR is no longer valid or desired, please feel free to close it. If no activity occurs in the next 30 days, it will be automatically closed.

[github-actions[bot]]

This PR has been closed since it has not had any activity in 120 days. If you feel like this was a mistake, or you would like to continue working on it, please feel free to re-open the PR and ask for a review.