Switch to attachments v4

[gferon]

which means dropping CDN0 which was AWS CloudFront and use CDN2 or CDN3 depending on what the server tells us to do.

TODO:

• Make sure we still have post_to_cdn0 functional, even though I'm not sure whether it is used or not.
• Add logic for CDN2 (we cannot know whether it is still in use or not, the server tells us to use it).

In general, I'd like to ditch libsignal-service-actix since it is not used anymore, which in turn will let us remove the PushService trait and let us have a much nicer direct access to the HTTP API.

Closes #320

[rubdos]

Some very minor suggestions (one is clippy, easily applied). This is very pretty code, well done.