Allow chromeless applications to render via non-/app routes

[eliperelman]

In order for an application to be chromeless, we need the ability to render that application via a route that may not start with /app.

• Make non-/app apps routable on the frontend
• Modify backend to allow rendering Kibana from non-/app route
• Explore implementation of a backend API that plugins can use to render the bundle for a specific application

Blocks #41981.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[joshdover]

This part will definitely be tricky due to the integration with the legacy code that we still have to support for now.

One suggestion I have is to maybe avoid replacing the legacy ui_render_mixin.js and instead duplicate the necessary parts for the time being, just so we can support something like:

router.get({ path: '/login' }, (context) => context.application.renderEntryPoint())

This way you can leave the default /app/* handling in Legacy, and avoid moving this to NP and providing some bridge for the legacy details that are still needed. It will mean some code duplication though, so we should explore doing it "the right way" first, but maybe timebox it so we don't lose too much time on making that work seeing how it will be removed completely pretty soon.

[pgayvallet]

I think there might be a missing step?

• Modify clientside appService / navlinkService to allow redirecting an app to a non /app/{appId} url ?