Add 'may-spotless-apply' profile

[Vlatombe]

I often find myself forgetting to apply spotless before committing.

On some repositories, I have set up a pre-commit git hook running mvn spotless:check. However, this has to be set up on each repository that has spotless check enabled, and disabled on those which don't.

I think this goes one step further, by defining a conventional maven profile may-spotless-apply that will run spotless:apply only if spotless is enabled for the given repository.

This allows any user to incorporate -Pmay-spotless-apply as part of their usual build command (or defined in their maven settings active profiles section), and have spotless:apply automatically called if the project has it enabled.

See also jenkinsci/plugin-pom#1017

Testing done

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jglick]

(same question as in plugin-pom)

[basil]

Thanks for the PR!