Add Async.withCancellation

# Add Async.withCancellation

I propose we add `Async.withCancellation`. This would allow using outside CancellationToken, such as the [RequestAborted](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.http.httpcontext.requestaborted?view=aspnetcore-2.1) from `HttpContext`. The problem with [Async.Start](https://msdn.microsoft.com/en-us/visualfsharpdocs/conceptual/async.start-method-%5Bfsharp%5D?f=255&MSPPError=-2147217396) is it does not provide a way to return a value.

The existing way of approaching this problem in F# is provided here:

https://gist.github.com/eulerfx/c41d50c6fba8e88cf16a21ed7c3c14bd#file-async-withcancellation-fs

Inlined:

```
let withCancellation (ct:CancellationToken) (a:Async<'a>) : Async<'a> = async {
let! ct2 = Async.CancellationToken
use cts = CancellationTokenSource.CreateLinkedTokenSource (ct, ct2)
let tcs = new TaskCompletionSource<'a>()
use _reg = cts.Token.Register (fun () -> tcs.TrySetCanceled() |> ignore)
let a = async {
try
let! a = a
tcs.TrySetResult a |> ignore
with ex ->
tcs.TrySetException ex |> ignore }
Async.Start (a, cts.Token)
return! tcs.Task |> Async.AwaitTask }
```

## Pros and Cons

The advantages of making this adjustment to F# are

* This would be easily discoverable to more user
* Potentially implemented in a cleaner way

The disadvantages of making this adjustment to F# are

* None I know of

## Extra information

Estimated cost (XS, S, M, L, XL, XXL): XS

Related suggestions: (put links to related suggestions here)

## Affidavit (please submit!)

Please tick this by placing a cross in the box:
* [x] This is not a question (e.g. like one you might ask on [stackoverflow](http://stackoverflow.com)) and I have searched stackoverflow for discussions of this issue
* [x] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate
* [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:
* [x] This is not a breaking change to the F# language design
* [x] I or my company would be willing to help implement and/or test this