Indicate the purpose of requests and responses passing through interceptors

[shockey]

Is your feature request related to a problem?

It's quite difficult to make decisions in request and response interceptors due to a lock of context on what the request is for.

Describe the solution you'd like

I'd like to see a purpose: String field added to requests that pass through our interceptors, with an enum of values: document, try-it-out, config, etc

Describe alternatives you've considered

The best way to do this today is try to interpret the purpose of the request based on the URL. This is brittle 😢

Additional context

We currently have loadSpec: true set when we're fetching a document - this is an extension of that idea.

We'd need to deprecate the loadSpec flag and drop it in v4, as well.