Rewrite pipeline builder

[streamich]

• Depends on: Expression AST builder #56748
• Generate AST instead of expression strings when building expressions for visualizations.
• Update / add more test coverage

Parent issue: #46909

[elasticmachine]

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

[lukeelmers]

coping @ppisljar notes over from duplicate issue #61829:

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

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

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

[lukeelmers]

The remaining work for this was started awhile back in #65396, but then put on hold while we finished work on the Expressions AST Builder which could be used in the pipeline builder.

@timroes and I discussed this offline -- Now that the app team is taking over the visualizations plugin, they will assess whether this refactoring is still necessary. After more of the visualizations have implemented the toExpressionAst method, there will be very little left in this file, and it might even be removed entirely.

To avoid stepping on the app team's toes, I will go ahead and close this issue as we no longer need to track it in the app arch expressions roadmap (plus it really has more to do with visualizations anyway).