Implement proxy endpoint for ElasticPress instant results

[roborourke]

The Instant Results feature that landed alongside Altis v11 is not currently fully implemented as I misunderstood what it did.

We need to implement a version of https://github.com/10up/elasticpress-proxy that works for Altis. Notably it cannot use the filesystem other than /tmp - and then it must also properly handle running on multiple servers and clearing the cache whenever the site is reindexed.

Acceptance criteria:

• [ ] Functionality of https://github.com/10up/elasticpress-proxy is replicated for Altis
• [ ] Config option for enabling instant-results feature
• [ ] Instant Results is switched off by default
• [ ] Docs added for new instant results feature including how to activate it and what it does - namely the search modal overlay. Add a screenshot as well using the twentytwentytwo theme and some demo content & tags
• [ ] Feature works on multisite
• [ ] DB queries can be cached

[veselala]

Hey team! Please add your planning poker estimate with ZenHub @robindevitt @roborourke @shadyvb

[roborourke]

@humanmade/altis-vulcan drop questions in here or in the product team chat if you have any and I'll answer them.

[veselala]

Hey team! Please add your planning poker estimate with ZenHub @ferschubert-hm @jerico @kovshenin @mikelittle @wisyhambolu

[CStukey]

@jerico We refined and estimated during Aug 10 refinement meeting.

[CStukey]

@jerico we refined and estimated this ticket during the Aug 10 refinement meeting.