enhancement: sankey diagram visualization

[mmguero]

I was excited to see gandtt charts added in the recent release.

Another great addition to the list of available visualizations would be a sankey diagram. Here are some references:

• Wikipedia
• Sankey diagrams in addition to Elastic.co's non-open-source Kibana in Vega
• Community plugin on GitHub (works up through Elastic 7.6.2 but not in new framework)

[Timc00k]

Please do what Elastic company can't do for a long time. The whole world is waiting for fantasty - nested field support for kibana.