Enable direct cancellation for IHttpForwarder

[Tratcher]

Fixes #1542

Some customers using IHttpForwarder directly have asked for the ability to pass their own cancellation token. Today they have to replace HttpContext.RequestAborted which can be error prone.

This uses a Default Interface Method to avoid a breaking change.

[MihaZupan]

While on the topic, should we augment ApplyAsync methods on transforms to accept CancellationTokens as well?

[Tratcher]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[Tratcher]

While on the topic, should we augment ApplyAsync methods on transforms to accept CancellationTokens as well?

Updated. How does that look @MihaZupan?

[MihaZupan]

Looks good.

We should also add CTs to the new extension methods we just added - https://github.com/microsoft/reverse-proxy/blob/main/src/ReverseProxy/Routing/DirectForwardingIEndpointRouteBuilderExtensions.cs. I can do that as a followup.

Nvm :)

[Tratcher]

We should also add CTs to the new extension methods we just added - https://github.com/microsoft/reverse-proxy/blob/main/src/ReverseProxy/Routing/DirectForwardingIEndpointRouteBuilderExtensions.cs. I can do that as a followup.

? Those are route table wire-up methods, there's nothing to cancel. The one place we do call the forwarder would only ever be cancelled by the RequestAborted token.