Extract jib plugin to its own subproject

[ryandens]

Summary

• Moves the com.ryandens.javaagent-jib plugin out of the plugin subproject into two new subprojects: jib-common (the Jib extension logic, shared with a future tiny-jib plugin) and jib (the Gradle plugin entry point that wires JavaagentJibExtension into Jib's plugin extension mechanism)
• Introduces JavaagentJibPlugin as a thin Plugin<Project> wrapper in the jib subproject, so jib-common can remain a pure library dependency for a future tiny-jib plugin
• Fixes task ordering: pluginUnderTestMetadata in :jib now explicitly depends on :jib-common:jar and :plugin:jar
• Removes the orphaned JibExtensionConfiguration.java that was left behind in the plugin subproject

Test plan

• ./gradlew :jib:build passes (unit + functional tests)
• ./gradlew :plugin:build passes (no jib references remain)
• ./gradlew build passes across all subprojects

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