Support RxJS as result type

[egandro]

Hi,

i'd love to have RxJS as result types in methods instead of Promise

http://reactivex.io/rxjs/

This should be configurable in the code gen or enabled by a parameter.

[lukeautry]

I think this is part of a bigger story to allow returning non-internal or non-project types, but probably won't make a specific integration for RxJS.

[nch3ng]

It's big, but I love the idea!

[aerabi]

It should not be hard to implement. We can easily cast Observables into Promises:

RxJS 6:

someObservable$.toPromise();