Use of draft Variants

[jyasskin]

We're looking at shipping a version of Signed Exchanges (WICG/webpackage#350; designed to expect more changes in the future, of course) that uses the draft Variants header. In https://tools.ietf.org/html/draft-thomson-http-mice-03#page-4, we specify that implementations of the MICE draft use digests named mi-sha256-##, so mi-sha256-03 for draft-03. Variants does not yet specify anything like this, which introduces a risk that we'll unintentionally introduce uses of the current draft into the wild, which could make it harder to change in the future.

I'll send a PR to suggest that draft implementations use HTTP headers named Variants-05 and Variant-Key-05 later this week, if you don't mind.

[mnot]

Sure.