[fetch] Add support for federated token binding to fetch (#30)

I'm proposing to add support for (federated) [Token Binding](https://github.com/TokenBinding/) to fetch.

I've written a requirements/problem-description document for a Token Binding DOM API: https://docs.google.com/document/d/1x_3KI_v35NQFEsAbGGjNcyTw3Waoy1mL3YfTaQbyyuM/view?rm=minimal&pli=1

While I have included a strawman proposal in the doc, I'd appreciate help from the fetch editors/implementers to figure out what the right approach is.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/30