Implement Remoting.Client utilities for file upload and download

[Zaid-Ajaj]

Basically a couple of (extention) functions on the client side of things:

• (instance: File).readAsByteArray() : byte[] so that a File coming from Html.input can be easily turned into binary content that can be sent to the server (upload)
• (instance: byte[]).download(filename: string) -> unit for file download once someone has hold of some byte[] returned from the backend, we might need to infer the content type based on the extension of the file.
• docs for uploading and downloading files because recently many people have been asking for this

readAsByteArray() might return Async<byte[]> (not sure) but that should be no problem, really

[Zaid-Ajaj]

Docs here https://zaid-ajaj.github.io/Fable.Remoting/src/upload-and-download.html

[kaashyapan]

Is it possible to monitor file upload/download progress with the remoting client utility ?

[Zaid-Ajaj]

@kaashyapan Not really with the way the client proxy works