feat(plugin-flattener): allow bringing m2o and o2o relations between models

[romain-gilliotte]

Definition of Done

General

• [ ] Write an explicit title for the Pull Request, following Conventional Commits specification
• [ ] Test manually the implemented changes
• [ ] Validate the code quality (indentation, syntax, style, simplicity, readability)

Security

• [ ] Consider the security impact of the changes made