add cancellationToken param to DownstreamWebApi methods

[h3rmanj]

Addresses #1739

Add optional cancellationToken param to DownstreamWebApi methods and extension methods allowing consumers the possibility to cancel the operations.

For now I passed the cancellationToken down to methods supporting it. I'm not sure if it would be better to call ThrowIfCancellationRequested ourselves between await calls.

[ghost]

All CLA requirements met.

[jmprieur]

@jennyf19. Let's take this in IDownstreamRestApi (@h3rmanj : this will be a new version of IDownstreamWebApi with a better API than that one which as grown organically)

[MauRiEEZZZ]

@jmprieur Is there any information about the timeline we can expect IDownstreamRestApi you mentioned?

[h3rmanj]

Implemented in 2.5.0