Adds optional callback on item planned to CommandListManager

[rr-tom-noble]

Description

• Adds an on_item_planned callback to Pilz's CommandListManager (solve and solveSequenceItems) to allow callers to respond to plan completion of an item
• Prerequisite for #3596
• Defaults to empty lambda for backwards-compatibility
• Adds test case checking callback is called

Checklist

• [x] Required by CI: Code is auto formatted using clang-format
• [x] Extend the tutorials / documentation reference
• [x] Document API changes relevant to the user in the MIGRATION.md notes (Backwards compatibility maintained)
• [x] Create tests, which fail without this PR reference
• [x] Include a screenshot if changing a GUI (No GUI changes)
• [x] While waiting for someone to review your request, please help review another open pull request to support the maintainers