Allow to re-save existing referenced entity

The MR adds configurable functionality to automatically save referenced entities when the referencing entity is saved. I heavily relied on AI to generate tests, fix test failures, and draft the initial solution. See the MR description for more details.

[...] another one that would help greatly: #2862574: Add ability to track an entity object's dirty fields (and see if it has changed)

Yep, I have a @todo in the code about that one :)

[...] what's the purpose of making this mechanism optional? Is it only for BC purposes (i.e. we could get into double-save situations in various contrib modules that already handle this themselves)

BC was one aspect, the other main one was performance: depending on the widget you are using, it may or may not make sense to ever re-save the referenced entity, especially until we have proper change-detection in place. Even a simple entity may have several entity references, so I was trying to reduce the performance impact as much as possible.

You definitely have a point about admin UX, though: I was imagining that modules providing widgets able to leverage this logic would automatically switch the save mode when their widgets are activated, however this may be problematic if multiple entity form displays activate widgets with different save mode requirements. Maybe we could do something similar to what @Berdir was suggesting in #3203913: Make EntityReferenceItem autosave but on the field item itself? E.g. a ::setNeedsEntitySave method setting a needsEntitySave property, defaulting to the ::hasNewEntity return value?

Maybe we could do something similar to what @Berdir was suggesting in #3203913: Make EntityReferenceItem autosave but on the field item itself? E.g. a ::setNeedsEntitySave method setting a needsEntitySave property, defaulting to the ::hasNewEntity return value?

I posted an implementation of this approach here:

The alternative approach above is being discussed in Slack.