[Embeddables] Migration System for Unsaved State

[ThomThomson]

Currently, after #162986 the Dashboard does not run clientside migrations for any state passed in via the URL, or for any unsaved state that was in the session storage.

The function that runs on Dashboard saved object load means that most of the time, when the embeddable create function is called on Dashboards there are no more migrations to be run. If for any reason there are migrations to be run at this point, the embeddable inheritance structure means they get immediately undone because the Container's child input is considered the source of truth. This could happen when:

• The Dashboard has state in the session storage. If a user has unsaved panels in their session storage, they refresh the Dashboard, and Kibana has updated, the Embeddable create method will detect if any new migrations have been registered and will run them, but the container will pick up the changes and immediately undo them.
• There are panels from an older version injected via the URL from a snapshot link. Again the migrations will get run, but will immediately get undone.

Implementing this back-propagation on the existing Embeddables framework would require ugly workarounds and changes that would affect the stability of the Embeddable platform. Once we move to a less prescriptive inheritance structure, this will not be a problem.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)

[ThomThomson]

This should be solved once #167429 is completed.

[ThomThomson]

Closing because this is already solved by the new embeddable system.