Add msgpack-jackson3 module for Jackson 3.x support

[komamitsu]

Summary

• Adds a new msgpack-jackson3 submodule (jackson-dataformat-msgpack3) that ports the existing Jackson 2.x integration to Jackson 3.x (tools.jackson namespace)
• Requires Java 17+ and Jackson 3.1.2; the module is excluded from the build on older JDKs so existing Java 8 users are unaffected
• Includes JMH benchmarks (src/jmh/java) runnable via ./sbt "msgpack-jackson3/jmh:run"

Key differences from msgpack-jackson

• Package namespace: com.fasterxml.jackson → tools.jackson
• TokenStreamLocation replaces JsonLocation; byte offset passed as columnNr
• Lightweight MessagePackWriteContext replaces SimpleStreamWriteContext

[Copilot]

Pull request overview

This PR adds a new msgpack-jackson3 submodule that ports the existing Jackson 2.x MessagePack integration to Jackson 3.x (tools.jackson namespace). It is published as a separate artifact (jackson-dataformat-msgpack-jackson3), requires Java 17+, and is conditionally included in the SBT build / CI / release pipelines so existing Java 8 users of msgpack-jackson are unaffected.

Changes:

• New msgpack-jackson3 module mirroring msgpack-jackson, adapted for Jackson 3 APIs (ObjectWriteContext/ObjectReadContext, TokenStreamLocation, ValueSerializer/ValueDeserializer, builder-style MapperBuilder/TSFBuilder, etc.) and including a lightweight MessagePackWriteContext replacement for SimpleStreamWriteContext.
• SBT build, CI workflow, and release workflow updated to compile/test/publish the new module only on JDK 17+, plus a new sbt-jmh plugin and JMH benchmarks under src/jmh/java.
• Numerous regression tests covering tricky edge cases (null keys, write-context state, dup-detection NPEs, root-scalar ordering, multi-root containers, writeNumber(String) NaN/Infinity, etc.).

Reviewed changes

Copilot reviewed 41 out of 42 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
build.sbt	Adds msgpack-jackson3 project, JMH plugin, Java 17 gating via isJava17Plus.
project/plugins.sbt	Adds sbt-jmh plugin.
.github/workflows/CI.yml	Skips msgpack-jackson3 tests on JDKs < 17.
.github/workflows/release.yml	Splits publishing so jackson3 is only published with JDK 17.
msgpack-jackson3/README.md	New documentation for the Jackson 3 module.
msgpack-jackson3/src/main/java/.../MessagePackFactory.java	Jackson 3 BinaryTSFactory implementation.
msgpack-jackson3/src/main/java/.../MessagePackFactoryBuilder.java	New TSFBuilder for the factory.
msgpack-jackson3/src/main/java/.../MessagePackGenerator.java	Port of generator; uses new MessagePackWriteContext, node-buffering model.
msgpack-jackson3/src/main/java/.../MessagePackParser.java	Port of parser to ParserMinimalBase; uses MessagePackReadContext.
msgpack-jackson3/src/main/java/.../MessagePackReadContext.java	Read context (replaces JsonReadContext).
msgpack-jackson3/src/main/java/.../MessagePackWriteContext.java	Lightweight write context.
msgpack-jackson3/src/main/java/.../MessagePackMapper.java	ObjectMapper subclass with new builder API.
msgpack-jackson3/src/main/java/.../MessagePackSerializerFactory.java	Custom serializer factory installing key serializer.
msgpack-jackson3/src/main/java/.../MessagePackSerializedString.java	SerializableString impl tolerant of null values.
msgpack-jackson3/src/main/java/.../MessagePackKeySerializer.java	Key serializer using MessagePackSerializedString.
msgpack-jackson3/src/main/java/.../MessagePackExtensionType.java	Extension type value + serializer.
msgpack-jackson3/src/main/java/.../ExtensionTypeCustomDeserializers.java	Per-type extension deserializer registry.
msgpack-jackson3/src/main/java/.../TimestampExtensionModule.java	Module serializing Instant as msgpack timestamp ext type.
msgpack-jackson3/src/main/java/.../JsonArrayFormat.java	AnnotationIntrospector forcing ARRAY shape.
msgpack-jackson3/src/main/java/.../PackageVersion.java	Hard-coded package version.
msgpack-jackson3/src/main/java/.../Tuple.java	Small Tuple helper.
msgpack-jackson3/src/test/java/...	Ported test suite plus new regression tests for Jackson 3 specific behavior.
msgpack-jackson3/src/jmh/java/...	New JMH benchmarks (read/write/UTF-8) and model classes.

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[komamitsu]

@xerial Can you take a quick look at the changes of .github/workflows/* and *.sbt?