Added an attribute service to embeddable start contract which provides a higher level abstraction for embeddables that can be by reference OR by value. Added an example that uses this service.
* Add embeddable via saved object example
* give todoRefEmbed a different name from the by value one
* fix types
* fix order of unmounting
Co-authored-by: Christos Nasikas <christos.nasikas@elastic.co>
* Add a new platform embeddable example plugin
* Remove extra hello world test impl.
* cleanup
* code review updates
* Change example to highlight and have parent filter out children
* Fix deep comparison of embeddable prop
* adjust help text
