Resolve "Support default positional and keyword arguments in meta-operations"

This MR adds the ability to define optional positional and keyword arguments to meta-operations:

!arg [0, some_default]
!kwarg [foo, {some: {other: default}}]  # can be arbitrary

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped

Related issues

Closes #242 (closed)