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

next-api-decorators/lib/createHandler.ts

Line 28 in 0b618bd

const instance = new cls();