Add support for optional attach parameter to disable artifact attachment

[peter277]

New feature, improvement proposal

Support for an optional <attach> parameter (defaulting to true) that controls whether the generated runtime image should be attached as a Maven artifact.

In some use it is desirable to build the runtime image without attaching it to the Maven project (e.g. producing jlink image as part of a subsequent jpackage step, or for separate distribution outside maven ecosystem). Having this feature would avoid conflicts during install or deploy phases, and reduce noise in the project's output artifacts.

[peter277]

Implementation submitted as part of pull request #621