[Feature Request] Accept CancellationTokens in IDownstreamWebApi

[h3rmanj]

Is your feature request related to a problem? Please describe. I'd like to be able to pass a CancellationToken to IDownstreamWebApi methods, allowing me to cancel requests and tasks. This is useful if you do multiple requests in your own API, but the request to your API was aborted (indicated by HttpContext.RequestAborted).

About every other interface by microsoft accepts a cancellation token, so I'd expect this one to do as well.

Describe the solution you'd like I've made a proposal PR #1734 which adds an optional CancellationToken cancellationToken = default param to every IDownstreamWebApi methods and extension methods.