[Maps] pre-populate custom color maps

[nreese]

Fixes #54650

Its always easier to edit something then build something from scratch. So, this PR will generate the color map when users select "Custom color map" for the first time. This will make user's lives much easier. For ordinal styling, the user will see the valid ranges for the data set. For categorical data, the user will see the top 9 terms.

[elasticmachine]

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

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 45b855f304d051c2954a531059d3267006d6d713

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@elasticmachine merge upstream

[jsanz]

Testing it works as expected but only the first time, if you change for another field the custom ramp is still showing for the previous field.

This is not wrong by itself, the problem is that then I can't see a way to populate again the custom ramp and I can't remove all the values.

Maybe it would make sense when changing the field to clean the custom ramp to get in the same state as if it was the first execution?

[nreese]

Maybe it would make sense when changing the field to clean the custom ramp to get in the same state as if it was the first execution?

I like that idea. For most cases, the fields are pretty dissimilar in values so clearing the customColorRamp makes sense when changing fields. What do you think @thomasneirynck?

[nreese]

Branch and PR got messed up, replacing with #55545