Allow Handlers to have an instance

[msrd0]

As of right now, we do not allow Handlers to have an instance, that is, their respective handle method must not use the self parameter. This might be inconvenient in some cases, where the handler depends on runtime-data that shouldn't be stored in static variables. A good example for this would be the OpenAPI handler that is currently implemented as a raw gotham handler rather than as a resource read-all handler since it needs the specification, and there is currently no good way to infer it without using a middleware.

We should allow handlers to take self or &self parameters so they can refer to runtime data.