Request for feedback and PSA: possible changes to fetch/XHR integration

Hello everyone, I wanted to point your attention to issue 756 and the associated PR 757 where we are considering a change to how the API integrates with `fetch` and `XMLHttpRequest`.

Today: In order for fetch/XHR requests to be eligible for the API, you need to manually add a `Attribution-Reporting-Eligible` request header, which for other integrations (e.g. which use `attributionsrc`) is added automatically by the browser.

After this change: Manually adding the header will no longer work to grant eligibility, instead you will need to use a first-class API surface built into `fetch` and `XMLHttpRequest` (see the PR for details). The `Attribution-Reporting-Eligible` header will always be added automatically by the browser if the request is truly eligible, unifying behavior across surfaces and making the presence of the header more meaningful. This will require code changes for existing integrations, but they should be straightforward / mechanical. Requests that manually add the header will still go out, they just won’t properly integrate with the API.

Please let us know if you have any feedback by responding to the issue or associated PR.