Plugin configuration changes lost during entity form submission

System information

• Drupal Core Version: 10.2+
• Menu Migration Version: 4.0.3
• PHP Version: 8.1+

Problem/Motivation

When creating export destination plugins (like AnotherMenu), configuration changes made in submitConfigurationForm() are not persisted. The entity form is directly copying plugin configuration from form state, overwriting any modifications the plugin made during submission.

Steps to reproduce

1. Create an export destination plugin that modifies configuration in submitConfigurationForm()
2. Example: Set $this->configuration['some_index'] = 'calculated-value'; in the plugin's submit handler
3. Submit the ExportType (Menu export) form
4. Reload the ExportType (Menu export) entity and examine the plugin configuration
5. The plugin's changes are lost; only the raw form values are saved

This applies to ImportType (Menu import) entities, and their ImportSource plugins as well.

Proposed resolution

Fixed by implementing EntityWithPluginCollectionInterface on ExportType and ImportType entities:

1. Created the AnotherMenu export destination plugin (needed for the menu cloning feature in a related ticket). This plugin clones menu items to another menu and requires complex configuration transformation in submitConfigurationForm(), making it an ideal test case for validating the fix.
2. Entities implement EntityWithPluginCollectionInterface and getPluginCollections()
3. Create custom plugin collection classes: ImportSourcePluginCollection and ExportDestinationPluginCollection
4. Entity forms manually sync plugin configuration after calling submitConfigurationForm()

This ensures that plugin modifications in submitConfigurationForm() are properly persisted to the entity.