Consider combining Attribution-Reporting-Eligible and Attribution-Reporting-Support headers

[johnivdel]

Currently, the app_to_web explainer proposes adding a new request header which specifies whether a request can register with either a web or OS provided Attribution Reporting API.

The EVENT.md explainer already adds an outgoing request header, Attribution-Reporting-Eligible, which specifies what type of registrations are possible for the response (event-source, trigger, navigation-source).

To simplify this, we could consider combining the two headers. Because both are already structured dictionary headers, it would simple to allow them to contain all of the boolean keys (event-source, trigger, navigation-source, os, web).

[apasel422]

One downside to combining the headers is that we expect users to manually set Attribution-Reporting-Eligible on fetch-based requests in order to support event-source registrations, but it is the browser that determines the os and web values. It may complicate the implementation for the browser to have to validate and/or modify the user-specified header to accurately reflect os and web.