[Maps] Add timestamp support for dynamic color / size ramps

[alexfrancoeur]

Today you can dynamically size and color of a point or shape. At the moment, we only accept numeric values. It would be great to support timestamps here as well. This would allow you to show a darker color for the most recent values and a lighter or transparent color for ones that have been around for awhile.

If we were to support this, which I've heard a few requests about already, we'd likely want to be able to support the new functionality we're thinking for color ramps. Inverting the colors (#34478) and custom color ramps (#27716). Custom color ramps might be an interesting challenge, as the component design is working on might not be a good fit for timestamps (#41603). Definitely worth a discussion.

[elasticmachine]

Pinging @elastic/kibana-gis

[alexfrancoeur]

Further more, it would be great if the future geo_line aggregation (elastic/elasticsearch#41612) could be dynamically styled in this fashion as well. One end of the line being light or transparent and the most recent part being darker.

cc: @talevy

[alexfrancoeur]

Closing as a duplicate of #39219