KAFKA-19465: feat(connect): add value.type config to InsertHeader SMT

[wilmerdooley]

This PR adds a new value.type configuration option to the InsertHeader SMT in connect/transforms/src/main/java/org/apache/kafka/connect/transforms/InsertHeader.java. The option lets users specify the Connect Schema type for the literal value being inserted as a header, supporting int8, int16, int32, int64, float32, float64, boolean, string, and bytes. When value.type is omitted, the existing behavior of using Values.parseString is preserved, so the change is backward compatible.

This addresses the underlying need behind KAFKA-10428 by allowing InsertHeader to produce schemas other than Schema.STRING_SCHEMA (most notably Schema.BYTES_SCHEMA), which is required for a ByteArray header.converter. Invalid value.type values are rejected at configure time with a ConfigException.

Testing strategy

Unit tests in connect/transforms/src/test/java/org/apache/kafka/connect/transforms/InsertHeaderTest.java cover the new behavior. insertionWithExplicitTypes exercises each supported type end to end through apply, asserting that the produced header carries the expected Schema and the expected value (with assertArrayEquals for the bytes case). A separate configRejectsInvalidValueType test verifies that an unknown type is rejected with ConfigException. The existing tests continue to pass because the default path (no value.type set) is unchanged.

JIRA: https://issues.apache.org/jira/browse/KAFKA-19465

[mimaison]

Thanks for the PR.

This is adding new configurations. These are considered part of the public API so it requires a Kafka Improvement Proposal (KIP). You can see the process on https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Improvement+Proposals

[wilmerdooley]

Thanks for taking a look, and for the pointer to the process. That makes sense, the new value.type config is a public-facing addition. I'll look into the KIP process for this change.

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