Allow transformer from observable to observable

[ShikaSD]

We have multiple cases in our codebase where we want transform element to multiple elements or apply some operators (throttling, distinct, etc) on it.
This PR allows transformer from element to observable.

[ShikaSD]

@zsoltk please comment and review

[zsoltk]

Any strong reasons for Observable over list? Maybe both?

Also can we just use overloads of using? I feel usingObservable is somewhat misplaced.

[ShikaSD]

@zsoltk

1. Observable allows for more variations over list + we will still use flatMapIterable on it, why not use it directly.
2. We cannot, because of generic erasure on lambda types, it considers both as a function over Function1.