Move plot function loading into BasePlotCreator

This MR moves plot function retrieval from the PyPlotCreator (previously: ExternalPlotCreator) to the PlotManager, making this a central aspect of plot creation. Furthermore, this makes BasePlotCreator a fully-capable creator that is agnostic to a specific plotting backend.

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
  • Reviewed & approved

Related issues

Closes #301 (closed), #303 (closed)