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.

Found an older issue with the same purpose: #3203913: Make EntityReferenceItem autosave → and another one that would help greatly: #2862574: Add ability to track an entity object's dirty fields (and see if it has changed) →

Started reviewing the current MR and I have an initial architectural question: 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)

If that's the only reason, I think the burden of this decision shouldn't be placed on the user (site builder) and it would be worthwhile updating those contrib modules (Inline Entity Form, Entity Reference Revisions, Media Widget) to stop auto-saving existing entities when the core functionality is detected instead.