Add Maven Central publishing for ir and helidon-extensions

[sku0x20]

Summary

• Adds com.vanniktech.maven.publish 0.37.0 to ir and helidon-extensions
• Shared config (signing, license, developer, SCM) in root subprojects block; each module only declares coordinates + pom { name, description }
• Version read from -PpublishVersion Gradle property (falls back to "rolling" locally)
• GitHub Actions workflow publishes both modules to Maven Central staging on every release

Test plan

• Merge PR
• Create a GitHub release — publish.yaml triggers and uploads both artifacts to Maven Central staging
• Verify version in staging, then manually release from the Sonatype portal

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