New platform should initialize the legacy rather than the other way around

[epixa]

The legacy KbnServer is currently responsible for initializing and running the new platform, which means when a user runs bin/kibana, the code path goes through the legacy cli code, starts up the legacy server runtime, and then from that the new platform code path is kicked off.

In order for the new platform to have greater control over the overall runtime and where legacy code falls within the overall lifecycle in Kibana, we should invert this. bin/kibana should invoke the new platform CLI code, which starts up the root system and gets everything in the new platform world going. Within the core system's start routine, we should start a legacy service that is responsible for running the legacy KbnServer code.

This also gives us the ability to inject new platform objects into the legacy world at runtime, which is how we can create temporary bridges from the old world into the new.

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See breakdown of tasks in #19994 (comment)

[epixa]

@elastic/kibana-platform