Allow 'validate' to write violations to a file, and not stop the build

[dhendriks]

Is your feature request related to a problem? Please describe.
We have various plugins that check various things during the build. If there is one issue, for instance a format issue, in one plugin, the build stops and we don't know what other problems exist. We have to fix it, and run the build again, and fix more issues, etc. Various Maven plugins support to not fail, but only warn, and to write violations to a file. We can then collect all of these at the end, and make a report out of those. The formatter-maven-plugin doesn't support such use cases yet.

Describe the solution you'd like

• Add an option to configure the result of the plugin in case of violations: either just report (new) or fail (current behavior).
• Add an option to write violations to a file, in addition to reporting them on the console.

Describe alternatives you've considered
I currently don't see alternatives.

Additional context
n/a

[ctubbsii]

One workaround for now is to just run the format goal, rather than the validate goal, and check to see if any files changed at the end of the build.