Proposal: Fold ES Admin Into This Plugin

The [ES Admin](https://github.com/alleyinteractive/es-admin) plugin allows developers to offload expensive admin queries (e.g., searching the media library against a postmeta field) to Elasticsearch. However, there is a lot of overlap between this plugin and that one (adapters, DSL class, faceted search interfaces, etc).

It would be nice to be able to add support for offloading admin queries to Elasticsearch as part of the configuration API exposed by this plugin rather than requiring another plugin to be installed, and we could capitalize on the fact that we've got adapters/DSL/etc in this plugin already. The disadvantage would be that this plugin currently only supports VIP Enterprise Search and ES Admin supports other search providers. The counter-counterargument is that two of the adapters that ES Admin supports (Jetpack Search and WPCOM) are no longer relevant, since VIP is dropping support for Jetpack Search in favor of VIP Enterprise Search and the WPCOM adapter only applies to sites on wordpress.com, which at this point basically don't exist due to VIP Go migrations. The important piece is the SearchPress adapter, which we already have an open issue for, and integrating ES Admin could be a good opportunity to just build out support for SearchPress in this plugin as well.

If ES Admin were folded into this plugin, we would need to expose a function to enable support for it and configure it, which will require some discussion.