Bug 1760663 - Add blocklist support for WebGPU. r=gfx-reviewers,nical

This patch just adds the plumbing to allow for baked in blocklist rules
or the downloadable blocklist to prevent certain configurations from
getting WebGPU. It does not add any rules.

It also changes us from allowing WebGPU only in nightly, including
tests, to not release and not beta. This allows try to run the WebGPU
tests as expected, since even try builds forked from mozilla-central are
not considered nightly builds by CI (or so it seems).

Differential Revision: https://phabricator.services.mozilla.com/D141682