Fix mypy default args issues in Workflow

[mshatkhin23]

Pull Request Checklist

• Towards fixing Create a hera mypy plugin #911
• Tests added
• Documentation/examples added
• Good commit messages and/or PR title

Description of PR
Fixes mypy linting issues with default args for mixin fields of arguments, metrics, volumes in workflow.py

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (76660e7) 80.3% compared to head (9520783) 80.3%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #918   +/-   ##
=====================================
  Coverage   80.3%   80.3%           
=====================================
  Files         49      49           
  Lines       3833    3833           
  Branches     779     779           
=====================================
  Hits        3081    3081           
  Misses       557     557           
  Partials     195     195           

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