[Maps] save custom color ramps

[johannamelly]

It would be great to be able to save custom color ramps (colors and thresholds), and then select it in the list like the default color ramps.

Sometimes we create a color ramp with a lot of different values and it can be time consuming in the case we have to use it in several maps on the same index.
Cloning a map can be a good trick in some cases, but it does not work all the time - for example if the concerned layer is the right source of term joins, and the left source changes.

[elasticmachine]

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

[thomasneirynck]

cc @stratoula @ghudgins fyi - this is a similar request in Maps

[teresaalvarezsoler]

@ghudgins I think you were working in a solution for this issue. It would be great if the solution could be centralize so we can use it in Maps - are you thinking about doing it this way?

[ghudgins]

yes - @stratoula has a proof of concept that would leverage saved objects for color stops / palettes.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)

[ThomThomson]

Blocked on #155037 - we will be using the Kibana implementation of color mappings to implement this functionality when it is available.