Refactor build user vars plugin: Remove unnecessary utility class, relocated constants to a more generic context

[fabiodcasilva]

Refactor build user vars plugin:

• Remove unnecessary utility class.
• Relocated constants to a more generic context class.
• Add missing tests to UsernameUtils

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] 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