This patch imports and implements all the infrastructure for origin
trial tokens, minus the crypto stuff / token verification.

We still don't hook it anywhere. The intended setup for now would be to
have the `OriginTrials` object hanging off the `Document` (or global
perhaps, not sure yet). That has a self-descriptive API to enable trials
(UpdateFromToken), and check enabledness status (IsEnabled).

There are some tests in the origin-trial-token crate
(third_party/rust/origin-trial-token/tests.rs). No test for the DOM code
yet because this isn't hooked into yet.

Differential Revision: https://phabricator.services.mozilla.com/D139033