[FLINK-39401][formats] Port raw line-delimiter option to release-2.1

[featzhang]

What is the purpose of the change

Port PR #27897 (FLINK-39401) from master to release-2.1. Extends the raw format with an optional raw.line-delimiter configuration that lets each Kafka/file message encode multiple records separated by a delimiter.

This is a clean cherry-pick with zero conflicts — the raw format code on release-2.1 has not diverged from master for the touched classes, so the backport is byte-for-byte identical to the master version (file content + tests + docs).

Brief change log

• RawFormatOptions: add LINE_DELIMITER ConfigOption (no default, supports Java escape sequences like \n, \r\n).
• RawFormatFactory: read the option, register it in optionalOptions(), and pass it to the (de)serialization schemas.
• RawFormatDeserializationSchema:
  • new 5-arg constructor accepting @Nullable String lineDelimiter; the previous 4-arg constructor delegates with null for backward compatibility.
  • pre-compiled Pattern for splitting; new deserialize(byte[], Collector<RowData>) override emits one RowData per segment.
  • null message with delimiter → zero rows; trailing delimiter stripped so round-trip produces one row, not two.
• RawFormatSerializationSchema:
  • new 4-arg constructor accepting @Nullable String lineDelimiter; old 3-arg constructor delegates with null.
  • pre-computed delimiterBytes; serialize() appends them after the value bytes. null row still returns null.
• Docs: docs/content/docs/connectors/table/formats/raw.md and the Chinese mirror are updated with the new option row.

Verifying this change

Added tests (unchanged from #27897):

• RawFormatFactoryTest.testLineDelimiterOption — verifies the factory wires the option through correctly.
• RawFormatLineDelimiterTest (new, 11 tests) — covers:
  • deserialize without / with \n / with multi-char / with GBK charset delimiters
  • null message, trailing delimiter, round-trip
  • serialize without / with \n / with custom delimiter, null row

Run locally against release-2.1:

./mvnw test -pl flink-table/flink-table-runtime -Dtest='RawFormat*'

Result: Tests run: 52, Failures: 0, Errors: 0, Skipped: 0 (11 new + 7 factory + 34 existing SerDe). All pre-existing tests continue to pass.

Also ran ./mvnw spotless:check -pl flink-table/flink-table-runtime → clean.

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): no (new option is additive, behavior unchanged when unset)
• The serializers: no
• The runtime per-record code paths (performance sensitive): yes — raw format deserialize/serialize. When the option is unset, the behavior and allocations are unchanged; when set, a pre-compiled Pattern and pre-computed byte[] avoid per-record allocation.
• 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 on the LINE_DELIMITER option + website docs (docs/content{,.zh}/docs/connectors/table/formats/raw.md), both ported from [FLINK-39401] Extend raw format to support line-delimiter option #27897.

[flinkbot]

CI report:

• 5a5f054 Azure: SUCCESS

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[spuru9]

Clean cherry pick. LGTM if CI don't break in a deps gap.