Let custom casts be instantiated by the container

[patrickhoogkamer]

I was a bit surprised I couldn't inject a dependency via the constructor of a custom casts class. I'm open to create a PR to add it but I would like to know if it would be considered before I put in the effort. Is there a specific reason it isn't already possible? It would add a global call to the container within a Model concern which doesn't seem optimal, but it would improve the usability of custom cast classes greatly in my opinion. Thoughts?