Add new protocol interceptor that allows delegating to URLRequestHttpJob

[ibash]

Is your feature request related to a problem? Please describe.
As a user of electron, I want to be able to intercept http requests and optionally send a response back. If I don't choose to send a response I want it to be handled by chrome's network stack.

Describe the solution you'd like
I'd like to add a new protocol like interceptFooProtocol that:

• Calls into a handler at various points in the request job (at the start of the request, after headers are constructed, when headers are received, etc)
• Allows optionally providing a http response for the job
• If an http response is not provided, delegates to URLRequestHttpJob in chrome

Describe alternatives you've considered

• Use interceptStreamProtocol and use the node network stack
  • The problem with this is that URLRequestHttpJob does a lot under the hood, and rewriting it in js is possible, but a lot of work. Among other things it adds headers, manages cookies, and checks ssl certs for pinning.
  • The other problem is that as bugs arise with interceptStreamProtocol it still needs to go through electron to be fixed, vs calling into URLRequestHttpJob and knowing that it will be handled the same as chrome.
• Use WebRequest to intercept
  • This only allows inspecting the request or cancelling it, you can't provide an http response as far as I can see.

Additional context

• This would be helpful in bringing offline support to nativefier.
• I've also been discussing this with Ryan Patterson who's working on chronicler (cc @CGamesPlay)

[ibash]

If there's consensus on what this can/should look like I'd be happy to take a stab at making a PR.

[maxosprojects]

+1

[nornagon]

This might be a duplicate of #15434?

[giuliopaci]

@nornagon I also think so.