Create option for config diff

Motivation

In a multisite setting, building feature or site splits is really hard because so much of the configuration has to be completely duplicated in each config location that it is represented in. If you have a default core.entity_form_display.node.page.default.ymland you want to change the order of the fields in a split, you need to have an entire copy of the config file, updated with new weights. This means that if you make some change to the default configuration and you want that to be represented in the split, you end up having to do a lot of file comparisons between the two versions, copying/pasting from the default config to the split config, then probably re-export the whole thing if something is out of order.

It would be ideal if there was an option for a split to only export the part of the configuration that is different. In the previous example, this would mean that you would have an exported config file that has the minimum information required to record the differences and nothing more.

A more common example of this is user.role.*.yml files. The Config Role Split moves in the direction of solving this problem for that one type of configuration, but the underlying issue still exists.

This idea may be related to https://www.drupal.org/project/config_split/issues/2934170

Proposed resolution

Add a filter that can read an incomplete config file and merge it with another configuration file. The filter would also be able to produce the incomplete config file for exports.

Remaining tasks

TBD

User interface changes

If this can be implemented through Config Split, I think this would potentially be a checkbox under conditional. Alternately, it could be another section of the form, similar to Complete and Conditional

API changes

TBD

Data model changes

TBD