ESQL: Refactor top-level query planning

[alex-spies]

ESQL's query planning currently uses multiple nested ActionListeners (see entrypoint here. Because of this, the code needs to be read last-to-first, starting with the plan optimization (last step), going deeper until the initial planning step (first step).

The ActionListeners seem to only contain procedural code, so this can probably be rewritten as simple procedural code: parse -> plan -> optimizations, resp. the number of action listener layers can likely be reduced.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)