visualizations: toExpression function

[ppisljar]

currently we build expressions for visualizations in build_pipeline.ts by concatenating strings. It has two problems:

• string concatenation is easy to break and hard to test
• impossible to extend (for 3rd party visualizations)

we should add toExpression or toAST function to visualization definition which accepts VisParams and returns correct AST.

parent issue: #59598

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[streamich]

Seems to be duplicate of #51246

[lukeelmers]

yes, i think this is a duplicate. will close.