JENKINS-46081: Add MessageExclusion trait

[chriskilding]

JENKINS-46081 - Add MessageExclusion trait

Add a corresponding trait for the MessageExclusion extension.

Checklist

• [x] I have read the CONTRIBUTING doc
• [x] I have referenced the Jira issue related to my changes in one or more commit messages
• [x] I have added tests that verify my changes
• [x] Unit tests pass locally with my changes
• [x] No Javadoc warnings were introduced with my changes
• [x] No spotbugs warnings were introduced with my changes
• [ ] I have interactively tested my changes

Types of changes

• [x] New feature (non-breaking change which adds functionality)

Further comments

I have tried to test this interactively by running a noddy multibranch pipeline job with an example Jenkinsfile, but I’m not sure that the MessageExclusion itself actually works.