Allow override of displays not build with display builder

Problem/Motivation

In 📌 [1.0.0-alpha2] Add content display overrides Active , the select list triggering the override (the one where we pick a UI Pattern's Source field) is related to the display builder activation in DisplayBuilderEntityViewDisplayForm.

So, we can override an entity view display only if it already use Display Builder.

The initial plan was to allow overrides for any entity view displays, even the ones built from Manage Display or Layout Builder, but it was not possible yet because we are not able to init a Display Builder display from Layout Builder.

Proposed resolution

Once ✨ [1.0.0-beta2] Migration from Layout Builder Active and ✨ [1.0.0-beta2] Layout plugins support Active are done, we can resume the work.