Allow setting Express Router as the custom api endpoint?

[Martinsos]

Right now, for custom api endpoint in Wasp, you need to provide a JS function that is Express middleware.

If would be cool if you could also provide an Express Router, which is (in a way) a higher-level construct, in the sense that it captures multiple api endpoints in it, that you can mount all together under a route. I am not sure if we should just allow registering it as a custom wasp api endpoint, or if we should add a special support for it at Wasp DSL level, next to custom api endpoint -> this deserves some consideration, and should be included with thinking in general about the custom api endpoint API design.