Add custom entity parent relationships

Problem/Motivation

There is no views support for custom entity parent relationships - only nodes have been added https://www.drupal.org/project/linkchecker/issues/3539894 ✨ Add parent entity relationship Active .

Steps to reproduce

Try to add a relationship to a custom entity in a Linkchecker View.

Proposed resolution

Provide views support for custom entity parent relationships as well as just nodes.