Provide NIO Futures TracingSupport extensions package (not sure how to call the module yet)

[ktoso]

It's quite common to want to quickly wrap some code in a span for debugging, like:

let work = tracer.withSpan(name: "collecting work") { 
  // collect work
  // do things here
  return work
}

we should likely offer this build in suggar right away, and then the same would be useful for NIO futures...

let work = tracer.withEventLoopFutureSpan(name: "collecting work", context: context) { 
  return  makeSomeCall()
    .map()
    .map()
    .map() 
}

There's a number of alternative API shapes this could take... promise.traced(context) etc as well...

Let's see which APIs we should provide in an NIO "tracing helpers" package.

[pokryfka]

This popped up previously: #98

For reference convenience methods I have been using:

https://github.com/pokryfka/aws-xray-sdk-swift/blob/master/Sources/AWSXRayRecorder/Recorder%2BHelpers.swift
https://github.com/pokryfka/aws-xray-sdk-swift/blob/master/Sources/AWSXRayRecorder/Recorder%2BNIO.swift