Issue #3547314: Add support for configuring specific entity types and their corresponding bundles and languages for Markdown conversion.

Issue #3547314: Add support for configuring specific entity types and their corresponding bundles and languages for Markdown conversion:

• Replace 'supported_entity_types' with structured 'supported_entities'.
• Add deprecation notices to API and schema (deprecated in 1.1.1, removed in 2.0.0).
• Introduce SupportedEntitiesConfigForm for bundle/language configuration.
• Add update hook (markdownify_update_10002) to migrate old config.
• Update validator interface and service for new structure.
• Add access checker service and form integration.

Closes #3547314

Expected Result on /admin/config/services/markdownify path: