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.
Added example for using dashboard container by value
1.1 Refactored embeddable explorer e2e test to use new example, removed not needed kbn_tp_embeddable_explorer plugin.
For embeddable explorer examples went away from using getFactoryById() to improve type checks
There is new component a replacement for EmbeddableFactoryRenderer with slightly more flexible api: EmbeddableRenderer.
3.1 We can improve it going forward to support more use case
* 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 plugin own contract as third element of getStartServices result
* adapt plugins code
* update tests
* revert unknown to use void again
* update generated doc
* fix UT
* update mock to allow non-object `pluginStartContract`
* add @typeParam documentation
* 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
