feat(backend): basic FGA model

[Dam-99]

What this PR does / why we need it:

Introduce a skeleton for the FGA model, to be expanded when all the requirements are defined.

Additional documentation e.g. usage docs, diagrams, reviewer notes, etc.:

The model in this PR is probably not in its final state, and will likely be reworked in the future, but it's a starting point to add FGA to Edgehog

---

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[lusergit]

This is a good starting point, we'll need to refine it for sure but it's ok 👍

[OmarBrbutovic]

Since the PR had more commits and they were probably squashed, the commit message got a bit messed up, could you please fix it