[Feature] support Stream -> Async<Stream>

[Zaid-Ajaj]

Although byte[] -> Async<byte[]> gets the job done, it is not ideal because it means data is allocated in memory. Supporting Stream makes more sense for file uploads and downloads:

• Stream -> Async<'T> for upload
• 'T -> Async<Stream> for download

Fable doesn't support Stream but I think we can work around that using extension methods like the ones we have for byte[].

TypeInfo from SimpleJson needs to be extended so that it knows how to work with Stream