DDC-2292: [GH-577] Allow extending EntityManager and use create method

[doctrinebot]

Jira issue originally created by user @beberlei:

This issue is created automatically through a Github pull request on behalf of ruian:

Url: #577

Message:

If you create your own entityManager by extending this one, you can't use ::create method because
it will create an instance of the last one and not our new entityManager

[doctrinebot]

Comment created by @beberlei:

A related Github Pull-Request [GH-577] was closed
#577

[doctrinebot]

Issue was closed with resolution "Invalid"