HTTP header customization when fetching the presentation page

[markafoltz]

It's possible for Web applications to add custom HTTP headers, i.e. for authentication, to resource fetch requests. We have an internal application that is looking to load an authenticated page as a presentation receiver page with a custom header.

The Fetch API does this by allowing the application to pass a Headers object with a Request.

We could make a similar improvement to PresentationRequest to allow passing headers at construction time, or perhaps with start().

This issue is a placeholder for discussion, I can formulate a more complete explainer if there is both internal and external interest in moving forward with this change.