Do not replace EntityTypeManager service but decorate it instead

Problem/Motivation

Currently the module overrides the EntityTypeManager service class to inject its functionality. To have it more fail-safe (e.g. if another module also tries to override the entity type manager), use the decorator pattern to extend the service instead.

Proposed resolution

• Use decorator pattern to extend EntityTypeManager service class - see these helpful links:
  • http://symfony.com/doc/current/service_container/service_decoration.html
  • https://www.phase2technology.com/blog/using-symfony-service-decorators-d...

Remaining tasks

• Provide a patch

User interface changes

API changes

Data model changes