[future] Add interfaces

[haringsrob]

Currently there are not many interfaces used.

Interfaces can help improving the api stability between major releases.

The plan is to add interfaces to all public facing classes such as:

• Repositories
• Models
• Controllers
• Requests

We need to add an interface for each of the traits we provide, for example in repository we can create the interface:

handlesMedias and implement the trait handleMedias

Ideally we provide an upgrade path for this. Perhaps we can do this using rector?