[Dashboard First] Add / Unlink Actions Require Specifically Shaped Input

[ThomThomson]

Problem:
The add to library action #73899 as it's currently drafted explicitly requires the input to be in the shape: { attributes: SavedObjectAttributesType }

Conversely, the unlink from library action will require the input to be shaped like a SavedObjectEmbeddableInput. These requirements are too strict, and not well defined.

** Possible Solution **
A byReferenceOrValue interface could be added which any embeddable which can be 'by reference OR by value' will implement: i.e.

export class BookEmbeddable extends Embeddable<BookEmbeddableInput, BookEmbeddableOutput>
  implements ReferenceOrValueEmbeddable<BookByValueInput, BookByReferenceInput> {

This interface would have to provide the ability to:

• determine if the implementing embeddable is by value or not
• transform any by value input for that embeddable type into by reference input (certain embeddables could show a save modal here)...
• transform any by reference input for that embeddable type into by value input

By creating this interface, the attribute_service becomes decoupled from the concept of the by value or by reference embeddable. Embeddables that have input in the specific shape required by attribute_service, could implement the interface by deferring back to the attribute service. I.e

getInputAsValueType = async (): Promise<BookByValueInput> => {
    return this.attributeService.getInputAsValueType(this.input);
};

[elasticmachine]

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