[Maps] Feature Request: have maps tile layers show loading status

[maihde]

Describe the feature: Map layers support the isLayerLoading function, which works great for ES-backed layers but doesn't work well for other layers (i.e. tile layers). MapBox provides an API for this situation via the isSourceLoaded and MapDataEvent but these are not tied into Kibana yet.

Describe a specific use case for the feature: When tile maps are slow to load, it is difficult to determine if tile layer isn't loaded yet or if the tile is simply void of data (i.e. fully transparent).

[elasticmachine]

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

[thomasneirynck]

+1 I like the concept. Accurate layer-status will become more relevant, as Maps starts to support more tiled data sources (e.g. upcoming vector tiles, next to exisitng support for wms, tms, ...).

[thomasneirynck]

This would be particularly useful for clients of the MapsEmbeddable as well #89302