Make instance entities aware of display buildables

#3562989: Implements RevisionLogInterface for Instance entity show some issues in our current management of "saved" data:

• we are storing data identical to, so duplicated of, the ones already stored by displays in config and content storage. This is useless, take storage space and error prone. What is happening if the data are diverging? how to reconciliation?
• this saved field has no equivalence in Revisions API

So, lets fix it before adopting Revisions API.

Proposed resolution

Instance entity must be aware of the buildable plugin

Add a buildable string base field to InstanceEntity. If an instance entity is created by a DisplayBuidable plugin, the plugin ID is stored in the base field.

Add PublishableInterface::getBuildablePlugin(): the plugin ID with the already stored contexts must be enough to load any DisplayBuidable plugin from any Instance entity.

Use the new mechanism in event subscribers

Thanks to the new ::getBuildablePlugin(), the logic of each DisplayBuilderEvents::ON_SAVE subscribers (1 per sub-module) become strictly identical, so we can do 3 simplifications:

• Merge them into a single subscriber service in main module.
• DisplayBuildable::getContextRequirement() can become protected, rely on plugins attributes, and be removed from the interface.
• We can pass the Instance entity coming from the controller in DisplayBuilderEvents instead of recretaing it from Instance ID.

We can do even more: remove the event all together ! We already have the instance, so we already know the buildable plugin, so we can publish.

Move the publication logic to display buildable plugins

In:

• Instance::saveIsCurrent() check the hash of the buildable sources and compare
• Instance::hasSave() becomes ::getPublishedHash()

Then:

• Remove save base field
• Remove Instance::setSave()

Remaining tasks

Leveraging the contexts may also be the opportunity to less rely on the "black magic" of parsing instance ID string. See: #3573905: Simplify DisplayBuildableInterface