Add new "attach" configuration parameter

[hgschmie]

This matches the attach parameter from the source plugin

Setting this parameter to false will skip attaching the generated jar artifact to the build. It will not be picked up by the install and deploy goals.

Omitting or setting this parameter to true will attach the jar to the build. This is the default behavior.

The use case for this is generating test jars as part of a build to be used in integration tests while they should not actually be deployed to a repository.

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