Custom visualizations plugin need to build their own expression AST

[timroes]

Plugin API changes

In Kibana all visualizations underneath render using Kibana expressions (that you can see user facing inside Canvas expression editor). Right now old custom visualization plugins are all using the same visualizations expression function underneath. We're going to remove this function in one of the upcoming minors, meaning your custom visualization plugin will require to register its own renderer and expression function and provide a toExpressionAst function.

You can check any of the PRs for core visualizations as a reference how those migration need to look like.