Add status config overrides with state

Problem/Motivation

In order to use Config Split to have different config per environment the status of the split is meant to be overwritten in different environments in order to have a split enabled or disabled. This is typically done in settings.php

Together with the UI it would also make sense to be able to turn on or off the split but only on the instance one is working on.
#3170692: Add UI for single split import operation
#3170694: Add UI for split deactivate operation

This override will of course in turn be overwritten by settings.php

State will also be used in config_environment to control which environment is active.

Proposed resolution

Add a config override system that uses state to turn the split on or off per instance only.
Add a drush command to set this state. Add form elements to various places (on the entity edit form, on the import and disable form)

Remaining tasks

implement
lots of new edge cases to test.

User interface changes

new radio options to locally override the status via state

API changes

None, state is not going to be an API

Data model changes

None, the state is external to the entity config.