Add interfaces, service, factory and validators

Problem/Motivation

The Drupal 11 upgrade task ( 📌 Automated Drupal 11 compatibility fixes for wordpress_migrate Needs review ) has additional new features that may not be necessary for Drupal 11 compatibility.

Proposed resolution

I'm suggesting we split off those aspects to this issue, and that will make the D11 update a smaller patch. (I am not certain which patch should be applied to 8.x-3.x first however.)

Remaining tasks

Develop patches.

User interface changes

None.

API changes

Additional factory, interfaces, service, validators.