Make RenderCompleteDispatcher reactive

[streamich]

• Make RenderCompleteDispatcher instances reactive—the should return an observable which reports the latest state of the element, whether it is still rendering, whether error happened, how may times it was rendered.
• Remove loading and error fields from EmbeddableOutput.
• Expose through embeddable the observable of RenderCompleteDispatcher that reports the state of rendering; and use that instead of loading and error fields of EmbeddableOutput.

See #74504 (comment)
Part of #71857

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)