Support for EP instant results custom proxy

[roborourke]

Reimplements the custom proxy from https://github.com/10up/elasticpress-proxy

Currently WIP, still to do:

• Implement request signing for cloud environments
• Come up with a way to cache the db query results that can be cleared on index/reindex so query only occurs on each container once. Will need to delete cache on reindex if possible

Fixes #410

Feature was revealed for v11 so should be backported to v11-branch.

[roborourke]

cc @stuartshields - it's functional like this on local but does a db query every time right now. Need to fix that and request signing / authentication by changing the curl stuff to use Guzzle directly. I'm away for a week so if you're able to work on it feel free, otherwise it'll be sometime the week after next before it's picked up again.