DDC-2384: [GH-639] Added abillity to use metacolumn as indexBy

[doctrinebot]

Jira issue originally created by user @beberlei:

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

Url: #639

Message:

Added ability to use meta column as indexBy. Useful if association entities is widely used.
Replace #204 PR

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-639] was closed:
#639

[doctrinebot]

Issue was closed with resolution "Fixed"

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-639] was closed:
doctrine/dbal#639