[DO NOT MERGE] Add opt-in Jackson 3 support via multi-release JAR

[cretz]

What was changed

• Added Jackson3JsonPayloadConverter as a multi-release JAR entry (Java 8 stub in src/main/java/, real implementation in src/main/java17/) that uses Jackson 3.x JsonMapper for JSON serialization
• Added JacksonJsonPayloadConverter.setDefaultAsJackson3(boolean, boolean) static method to globally opt in to Jackson 3 delegation, including for the default converter in DefaultDataConverter.STANDARD_PAYLOAD_CONVERTERS
• Jackson 3 converter supports two modes: native Jackson 3 defaults (jackson2Compat=false) and Jackson 2 compatibility mode (jackson2Compat=true via JsonMapper.builderWithJackson2Defaults())
• Converters with an explicit ObjectMapper passed to the constructor are not affected by the global Jackson 3 delegate
• Both converters produce identical "json/plain" encoding type and are wire-compatible in both directions
• Jackson 3 is published as an optional dependency with version range [3.0.4,) in the POM
• Added jackson3Tests test suite (8 tests, Java 17 launcher) covering direct converter usage, wire compatibility, global delegation via GlobalDataConverter.get(), and explicit-mapper opt-out
• Added Jackson-3-absent tests to the main test suite verifying correct error behavior on Java 8 (UnsupportedOperationException) and Java 17+ without Jackson 3 (NoClassDefFoundError)
• Added jackson3Tests CI step to the unit_test_jdk8 job
• Added @Documented to the @Experimental annotation

DO NOT MERGE - we are going to wait on Spring Boot 4 support at the same time to at least get reviewed at #2786

Checklist

1. Closes Jackson 3 support #2746

[Quinn-With-Two-Ns]

What about the SDK's internal uses of jackson like for Nexus?