[FLINK-38189][core][python] Add RowFieldExtractorSchema for Row field serialization

[Nflrijal]

What is the purpose of the change

This pull request introduces RowFieldExtractorSchema, a new SerializationSchema implementation that extracts and serializes a specific field from a Row object. This addresses the common use case where users need to serialize different fields of a Row separately, particularly for Kafka producers where keys and values require independent serialization.

Previously, users had to implement custom serialization schemas or use workarounds to extract individual Row fields. This change provides a reusable, type-safe solution that simplifies the common pattern of using one Row field as a Kafka key and another as the value.

Brief change log

• Add RowFieldExtractorSchema class in flink-core that implements SerializationSchema<Row>
• Add RowFieldExtractorSchemaTest with comprehensive unit tests covering:
  • Basic field extraction and serialization
  • Invalid field index handling
  • Null value handling
  • Empty Row handling
  • Multiple field extraction scenarios
• Add Python bindings in pyflink.common.serialization.RowFieldExtractorSchema
• Add Python unit tests in test_serialization_schemas.py
• Add Kafka integration test demonstrating real-world usage
• Add Javadoc and Python docstrings with usage examples

Verifying this change

This change added tests and can be verified as follows:

Java tests:

• Added RowFieldExtractorSchemaTest with 6 test cases covering normal operation, edge cases, and error handling
• Run with: mvn test -pl flink-core -Dtest=RowFieldExtractorSchemaTest

Python tests:

• Added 13 unit tests in test_serialization_schemas.py::SerializationSchemasTests::test_row_field_extractor_*
• Added integration test kafka_test.py demonstrating Kafka key/value serialization
• Run with: pytest pyflink/common/tests/test_serialization_schemas.py -k RowFieldExtractor -v

Manual verification:

• Validated end-to-end with real Kafka cluster
• Verified key/value extraction with multiple Row fields
• Confirmed UTF-8 encoding works correctly

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 (new @Public(Evolving) class added)
• The serializers: yes (adds new serialization schema)
• The runtime per-record code paths (performance sensitive): no (only used when explicitly configured)
• 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? JavaDocs / Python docstrings with usage examples
  • Javadoc includes detailed class description and usage example with Kafka
  • Python docstring includes parameters, return types, and example usage
  • Test cases serve as additional usage documentation

[flinkbot]

CI report:

• 249735f Azure: SUCCESS

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