cd.yaml template: Enhance with validate_only config option

[mhoffrog]

@jglick @timja

Description

This PR is following up PR jenkins-infra/github-reusable-workflows #32

The cd.yaml template is enhanced as follows:

• Add input selection for the manual trigger to provide the validate_only option of jenkins-infra/github-reusable-workflows/.github/workflows/maven-cd.yml
• Add appropriate inline help comments for individual use case config options

Testing done

see action runs with unleash-plugin cd.yaml:

1. automtically triggered: https://github.com/jenkinsci/unleash-plugin/actions/runs/10956424587
2. manually triggered: https://github.com/jenkinsci/unleash-plugin/actions/runs/10956762080
3. Screenshot of manual trigger start - updated after description format improvement commit:
   

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