[CHORE] Improve argilla-server folder structure

[jfcalvo]

We should improve folder structure on argilla-server with the following:

/api
  /handlers
    /v1
    /v2
  /policies
    /v1
    /v2
  /schemas
    /v1
    /v2
/contexts
/models
...

So the idea is to have everything related with the API inside an api folder. Inside that folder we should have handlers, policies and schemas.

For stuff not directly related with the versioned API we can have folders outside of api, like /context and /models.

Because these are changes that are not really complex to do and in order to avoid conflicts with current changes I would do this issue over branch feat/v2.0.0.

cc @frascuchon