Run current UI on top of new platform server plugin

[kimjoar]

While we're moving towards new platform we need to be able to first run server on new platform, then move UI over (as the UI parts of new platform aren't ready yet).

There are a couple potential options, e.g.

1. Moving the index.js in core plugins (e.g. https://github.com/elastic/kibana/blob/master/src/core_plugins/kibana/index.js) over to new platform and being able to handle the UI parts of that setup, e.g. uiExports.
2. Run separate plugins. So we'd have one plugin for UI and another for server, if that works.

[epixa]

I'm all for preserving compatibility with uiExports in the short term to help transition old code to the new platform, but we really need to kill uiExports entirely. The new platform is all about declaring which systems you depend on and receiving contracts for those systems. uiExports is the opposite: it's about a plugin declaring what functionality it has and hoping that something upstream will convert that into calls to an explicit contract.

[kimjoar]

++ definitely agreed. My idea here was mostly the transition period. We should build this so we can move away from it when we're able to more to new platform in the UI.

[epixa]

Closing this in favor of #16253. They're not exactly the same issue, but we'll learn a lot in the course of that issue, and it could make this moot.