Add CROSS_ORIGIN_EMBEDDER_POLICY and CROSS_ORIGIN_OPENER_POLICY constants

[ssokolow]

I'm currently building a site using actix-web, which relies on the http crate for strongly-typed header constants, and I noticed that there are a couple of headers which don't yet have constants here, forcing me to resort to manually specifying them as strings.

Specifically:

• Cross-Origin-Embedder-Policy
• Cross-Origin-Opener-Policy

As these headers need to be set to enable access to browser features like SharedArrayBuffer and un-throttled performance timers, I think it'd make sense to add CROSS_ORIGIN_EMBEDDER_POLICY and CROSS_ORIGIN_OPENER_POLICY constants.

[djc]

Since these are standardized in the HTML living standard, just submit a PR.

[ssokolow]

Right now, it's hard to find time for "annoying, but not show-stopping" stuff like this, so I'll have to just put it on my TODO list.

(As in I'll add a // TODO: Pending hyperium/http#460 comment in my code and defer it until my next clean-up sweep.)

[dekellum]

Ooops, sorry, I got header constants and status code constants confused, folks. Please ignore me.

~~Are these now also in the IANA list? That list is the only one claimed to be completely covered here. See #453 for background (also a good template for a new PR, I hope).~~