Load splits from a file in addition to config entities.

Problem/Motivation

Splits are managed as config entities, this allows for a "nice" user interface and integrates nicely with config overrides.
But config entities are managed (created, updated and deleted) via config import and config split acts on these imports. In order to use splits that are part of the import we load the splits from the transformed storage on import #3219993: Apply split changes in a single config:import execution on first enable →

However, this highlights that something is not ideal in the design of this.
It would be much clearer if the split configuration was committed to the code in its dedicated file in the codebase rather than deployed alongside the normal configuration.

Proposed resolution

Read split configuration from a config_split.yml file in the settings directory, (and apply normal config overrides as we do for the config read from the sync storage).
In a follow up issue we can create a UI to help manage this file and validate it etc.

Probably the easiest format for this file is simply a key called splits with an array of configuration that is now part of the config entity.

Remaining tasks

1. Implement
2. document
3. commit
4. celebrate

User interface changes

none. We will create a UI in a (to be created) follow up issue.

API changes

new place to define splits.

Data model changes

none