Migration from Layout Builder (content overrides)

Problem/Motivation

Once 📌 Improve content display overrides Active and ✨ [1.0.0-beta2] Migration from Layout Builder Active are done, we will be able to execute the migration also for content overrides.

Proposed resolution

When an Display Builder override is initialized, there are 4 possible sources of data and we load the first in this order of priority:

1. Field settings default value: the first, not because we want but because it seems Field API is forcing it. It may be the opportunity to challenge that.
2. Existing Layout Builder override (only for default display): priority because we keep existing content
3. Display Builder entity view display config: the most usual and expected situation.
4. Existing Layout builder config (if we do 📌 Allow override of displays not build with display builder Active )