Change Layout Preparation into an Event to allow proper alterations

Problem/Motivation

Currently the LayoutBuilder Render element itself holds the key to the layout preparation, making it difficult for storage plugins or custom code to decide how to ouput a layout

Proposed resolution

Implement an Event for preparing a layout so subscribers can easily alter the sections.

Remaining tasks

• Tests? Does this need to be tested outside of current test cases?

User interface changes

None

API changes

New LayoutBuilderEvents::PREPARE_LAYOUT event for modules to interact without layouts during rendering.

Data model changes

None

Release notes snippet

New LayoutBuilderEvents::PREPARE_LAYOUT event for modules to interact without layouts during rendering see https://www.drupal.org/node/3160587 →