[FLINK-37913][table] Add built-in OBJECT_OF function

[raminqaf]

What is the purpose of the change

This pull request implements the OBJECT_OF function as specified in FLIP-520 to enable creation of structured types in SQL and Table API. The function allows users to construct structured objects from key-value pairs without requiring UDFs, making structured type creation SQL-serializable and improving the overall usability of structured types in Flink.

Brief change log

• Added OBJECT_OF function definition to BuiltInFunctionDefinitions
• Implemented ObjectOfInputTypeStrategy for input validation (odd argument count, string keys, unique field names)
• Implemented ObjectOfTypeStrategy for output type inference (creates structured types)
• Added ObjectOfFunction runtime implementation that creates RowData from key-value pairs
• Enhanced Expressions.java with objectOf() methods for Table API support
• Added Python API support with object_of() function in expressions.py
• Updated SQL functions documentation with OBJECT_OF function details

Verifying this change

This change added tests and can be verified as follows:

• Added unit tests for ObjectOfInputTypeStrategy to validate argument count, type validation, and field name uniqueness
• Added integration tests in StructuredFunctionsITCase to test end-to-end functionality
• Verified SQL usage: SELECT OBJECT_OF('com.example.User', 'name', 'Bob', 'age', 42)
• Verified Table API usage: objectOf(User.class, "name", "Bob", "age", 42)

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 (additions to Expressions.java and BuiltInFunctionDefinitions)
• 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 / JavaDocs (updated SQL functions documentation, comprehensive JavaDocs for all new classes and methods, Python API documentation)

[flinkbot]

CI report:

• a49b6ad Azure: SUCCESS

Bot commands

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

[gustavodemorais]

I think there's a checkstyle violation making the CI fail. Make sure to run ./mvnw checkstyle:check locally

[raminqaf]

@twalthr There is also another case that I have prepared, but so far, it has been stashed on my branch. In the FLIP-520, you mentioned under the table API, this signature:

Expressions.objectOf(DataType, Object... kv);

In SQL, this would be equivalent to:

SELECT OBJECT_OF(OBJECT_OF(...)));

Is this a case we should also support in SQL? Does this mean that we can construct structure types from structured types?

[twalthr]

Thanks for addressing all comments. LGTM.