[Example] Embeddable by Ref or Value

[ThomThomson]

To support the upcoming initiative of reference and value embeddables in the dashboard first project, an embeddable example should be added containing demos of the following functionalities.

• The ability to add an embeddable to a container by value
• An embeddable that uses a union type for it's input to be either by reference OR by value
• A generic utility to unwrap any by reference input into the related by value input.
• The ability to convert a 'by reference' embeddable into 'by value' and vice versa from the container.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[ThomThomson]

Closed due to merging of #68719