[Maps] [Embeddables] Provide ability to know when map/embeddable is loading

[spong]

In integrating maps into the SIEM App with the Embeddables API, we would like to be able to provide our own custom loading indicator when the map is fetching data.

This is an enhancement request to expose some sort of trigger/action we can listen for to know when the map has started/finished loading. Currently, granularity of which layer is loading does not matter, so a global loading flag should suffice (but more is always better :)

I don't believe there is any need to know when sub-components are fetching data (e.g. _loadFeatureProperties() within tooltips).

cc @stacey-gammon @streamich @nreese

[elasticmachine]

Pinging @elastic/kibana-gis

[nreese]

@spong Can this be closed?

[spong]

@nreese, we were able to get the desired behavior via your additions from #47157, so this isn't a hard requirement on the SIEM side anymore. I still think this would be a nice feature for embeddables as a whole, but feel free to close since it's more geo-specific.

[nreese]

Closed by #47157.