Allow to provide the handler instance

[snordquist]

Currently, I try to use next-api-decorators in combination with tsyringe for dependency injection.

In order to get this to work I had to allow the container instance to instantiate the handler class.

(e.g. container.resolve(cls) instead of new cls())

It would be nice if next-api-decorators would allow me to provide the instance

https://github.com/instantcommerce/next-api-decorators/blob/0b618bd8d43b4d47b3ac3f8d50bd77d3971bc6e3/lib/createHandler.ts#L28