change filter to optional parameter in Job DSL

[limejuny]

Detail

• Fix #83
• The Expression (regexpFilterExpression) and Text (regexpFilterText) is optional field in Web UI, but there are required parameter in Job DSL (because constructor use those params.)
• Excluding these two fields from constructor, and adding setter.
• Tested with 1.87.0 and this new version.
  • using 1.87.0, in Job DSL API Reference - the field is required -
  • using this commit, in Job DSL API Reference
    • the field is not required
    • 
  • using 1.87.0, without regexpFilterText, regexpFilterExpression - -
  • with this commit, without regexpFilterText, regexpFilterExpression
    • 
  • with this commit, with regexpFilterText, regexpFilterExpression -

Testing done

### 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
- [x] Link to relevant issues in GitHub or Jira
- [x] Link to relevant pull requests, esp. upstream and downstream changes
- [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[limejuny]

Hi, @tomasbjerre could you review my code?

and if you ok, could you label this PR hacktoberfest-accepted or set hacktoberfest on the repository topic? I am participating in hacktoberfest, and I will be happy if this PR is accepted as a hacktoberfest's contribution. (https://www.jenkins.io/events/hacktoberfest/#quick-start)