Use header convention for identifying request

[jonathonherbert]

At the moment, we store the requestId for typerighter match requests in the body of the JSON we send.

The method we use elsewhere is an X-GU-LoggingContext header. See https://github.com/guardian/flexible-content/blob/master/flexible-content-common/src/main/scala/com/gu/flexiblecontent/util/LoggingContext.scala and https://github.com/guardian/flexible-content/blob/master/composer/src/js/app.js#L393-L419

It'd be good to use this convention for ease of debugging across projects.

We could also include contentId there, and sessionId if we can access it.