Make MD conversion service pluggable

Problem/Motivation

Right now we have hardcoded the League HTML-to-Markdown library. There might be other libraries or services people prefer for the task, most prominently AI services.

Proposed resolution

• Make the conversion service pluggable, both for local libraries and remote services.
• Expose the choice of conversion service in the admin UI (see https://www.drupal.org/project/markdownify/issues/3512589 ✨ Add an admin page for module parameters Active )
• Add service-specific settings to a subtab per supported service.
• Unknown (fresh) services default to no subtab and users have to figure out themselves how to send settings and preferences to the conversion service.

Remaining tasks

User interface changes

API changes

Data model changes