[Plan] Leverage temporary workspaces for entity creation / update

Problem/Motivation

Currently there is a huge problem space connected with unsaved entities to be created or updated:
- Ctools created a temporary storage for Views and PageManager
- Inline Entity Form maintains its own storage
- Node preview has one
- Entity Browser has one
- ...

And still...:
- When altering an entity form, there is no simple way to get the (unsaved) parent entity of a paragraph.
- There is no unified API for the storages above.

Proposed resolution

For creating or updating entities in an EntityForm, use a temporary workspace (added in #3095414: Add the concept of workspace providers).
Get Content Locking for free, for better editor experience.
Get preview for all entity forms for free.
Get better DX and maintability.

Remaining tasks

tbd