[Maps] Allow embeddable clients to check/get notified on loading state

[thomasneirynck]

Describe the feature:

Clients of the MapEmbeddable can get notified of loading-state (or be able to retrieve loading-state) of an embedded map.

Describe a specific use case for the feature:

Show a loading spinner while the map and layers are initializing.

This would be useful for applications where the map is embedded as part of a larger dashboard. E.g. the anomaly-explorer in ML. You don't want to present an incomplete map, when not all the data has loaded yet.

cc @alvarezmelissa87

[elasticmachine]

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

[thomasneirynck]

The main issue here that we need to be able to accurately determine loading state for tile-layers as well. This is also a blocking-feature for tiled layers (e.g. layers backed by vector-tiles) See also #76237.

[nreese]

The MapEmbeddable exposes a setEventHandlers method that allows for the setting of callbacks that are triggered on layer load events. These should provide the functionality requested by this issue.

https://github.com/elastic/kibana/blob/master/x-pack/plugins/maps/public/reducers/non_serializable_instances.d.ts#L22

interface EventHandlers {
  /**
   * Take action on data load.
   */
  onDataLoad: ({ layerId, dataId }: { layerId: string; dataId: string }) => void;
  /**
   * Take action on data load end.
   */
  onDataLoadEnd: ({
    layerId,
    dataId,
    resultMeta,
  }: {
    layerId: string;
    dataId: string;
    resultMeta: ResultMeta;
  }) => void;
  /**
   * Take action on data load error.
   */
  onDataLoadError: ({
    layerId,
    dataId,
    errorMessage,
  }: {
    layerId: string;
    dataId: string;
    errorMessage: string;
  }) => void;
}

[thomasneirynck]

Closing. with #76237 enablea the loading-spinner for tile-layers, and the setEventHandlers method allows for monitoring loading-state.