[Maps] [Embeddables] Provide ability to retrieve number of features for a given layer

[spong]

Feature request

This feature request is for providing the ability to retrieve the number of features for a given layer from a Map Embeddable. Similar to #43695, which requests the ability to know when a layer is loading, this would allow those working with Map Embeddables additional flexibility in providing user-facing features around layer state.

Use case

The impetus for this came from #47706, which aims to add targeted documentation for how to configure line data on the SIEM Pewpew Map. Knowing whether or not the line layers had any features would allow us to programmatically display an additional documentation/configuration UI element instead of having to show it at all times.

Implementation

Similar to the custom tooltip implementation (#46150), an onLayerStateChange callback could be provided to the MapEmbeddableFactory. E.g.

interface LayerState {
  name: string;
  id: string;
  isLoading: boolean;
  isHidden: boolean;
  resultCount: number;
  fitToData(): void;
  hideLayer(): void;
}

const onLayerStateChange = ({ layerStates }: LayerState[]) => {};

const mapEmbeddable = await mapEmbeddableFactory.createFromState({
  state,
  input,
  parent: undefined,
  renderTooltipContent,
  onLayerStateChange,
});

Note: I included the fitToData/hideLayer actions as well, but perhaps these are better fit to be called via the Embeddables Action API?

cc @nreese @stacey-gammon

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)