Trigger changesets should be extracted from Realm Management into Astarte Core

[rbino]

Right now, external Elixir applications are able to correctly validate Astarte Interfaces using the changesets contained in Astarte Core.

However, they are not able to do this with Triggers because the top Trigger changeset lives in Astarte Realm Management. This should be moved here so that it can be reused for, e.g., API clients installing triggers and so on.