[Maps] [Embeddables] Enable ability to provide custom renderer for tooltip

[spong]

In integrating maps into the SIEM App with the Embeddables API, we've come up with a few features that would require a custom tooltip when selecting a feature on the map. Most notably, having the ability to send data to the Timeline would be extremely beneficial to an analyst as they could use the map to not only filter and find data, but to also easily add it to an ongoing investigation. Being able to provide a custom renderer for tooltips would allow us to build out that functionality, or even provide custom field/row renderers for more easily groking specific events as we do in the Timeline now.

Expected inputs would similar to what the current FeatureTooltip provides, which includes a list of layers, and list of features. Having the ability to fetch a feature's properties similar to _loadFeatureProperties() would be extremely helpful as well :)

[elasticmachine]

Pinging @elastic/kibana-gis