Add context methods, and ability to pass transport to adapter for tracing (and context cancelation)

[deefdragon]

Would it be possible to create variants of the IPP (and perhaps other) client methods that consume context.Context as an argument? IE PrintFileCtx? and pass that context to the associated web-requests that get made?

As well as taking in a http.RoundTripper to pass to the transport in NewHttpAdapter?

I have my services instrumented with tracing to make sure the requests are getting executed properly, and without these two inputs, the tracing is nowhere near as robust. The individual traces for the web requests not even associated with the original caller.

I was able to get some tracing by manually copying the HttpAdapter and having it accept a transport argument, but without the context passed to the web requests, the tracing cant tie the executed http requests to the original call.