Change key actions to be bitflags

Currently, the Meilisearch action keys are single numbers and therefore cannot be combined. It's not possible to OR them NOT them. I needed to do a basic documents.add | documents.delete for the edit documents by using a function feature.

We must use bitflags to define those actions and be able to combine them. We could have 32 or 64 different combinable actions. We already use this library in heed and it works very well.