Remove Guzzle in favor of WordPress core functions

[ethanclevenger91]

Closes #445

This PR is probably incomplete. A couple notes:

• Moves away from Guzzle exception classes. Replaced with what I considered sensible alternatives, but this will alter the logs that result from failed requests
• Inexperienced with stubbing WordPress core functions, so you may desire more comprehensive testing
• Docker image wouldn't build w/o adding an argument to apt-get update

But that said, all tests pass and I'll be giving this a go on a staging client site.

[ethanclevenger91]

Oof, yeah, looking at the commit history, looks like some of this logic is part of some alternative centralized bunch of code that gets split between all your various plugins, so this probably isn't going to work super well from a workflow perspective, though it seems to be working fine standalone.

[jacobbednarz]

Thanks for this @ethanclevenger91! Luckily, I had combined cloudflare-backend not too long ago, so there aren't any additional plugins that rely on this now. The frontend is still separate however, there are some plans in the works to clear that up as well once we find some time for it.

Testing this locally looks pretty good to me and I can't stop anything that is out of what we would expect (even with exceptions). I'm happy to land this and if anything else pops up from it, we just address it in an upcoming release.