Custom types to be auto-retrieved from the State

[msrd0]

As of right now, our derive macro for the methods handle predefined names such as conn or auth. It would be nice if this was allowed for custom types through a trait or similar. This might be easy for auth, but more complicated for conn as the database connection requires wrapping the body, which might make this impossible to completely replace the current types, but would at least allow users to add own types.