Does not compile with Vapor 4 RC

[jhoogstraat]

No description provided.

[jhoogstraat]

As part of this PR the following improvements can be made:

Add case patch to OperationType

Especially relevant when patching/updating is done partially (e.g. only providing the fields to-be-updated).

Add the following extension:

extension QueryEndpoint {
    public func register(to routes: RoutesBuilder) {
        switch operationType {
        case .get:
            routes.get(use: handler)
        case .post:
            routes.post(use: handler)
        case .put:
            routes.put(use: handler)
        case .patch:
            routes.patch(use: handler)
        case .delete:
            routes.delete(use: handler)
        }
    }
}

This allows most routes to drop their implementation of register()

Update Custom to allow for different return values.

Right now T must be returned. This would implicate that the path parameter must be optional.

public final class Custom<T: CorvusModel, R: ResponseEncodable>: QueryEndpoint {
/// The return value of the `.handler()`.
    public typealias Element = R

//...
}