ESQL: ProjectAwayColumns rule not handling synthetic attribute correctly

[alex-spies]

The logical optimizer rules PushdownEval, PushdownRegexExtract and PushdownEnrich create temporary attributes when an EVAL, GROK/DISSECT or ENRICH is pushed past a SORT with conflicting names; e.g. SORT x | EVAL x = y essentially becomes EVAL $$x = x, x = y | SORT $$x | DROP $$x.

The temporary attribute $$x should be synthetic, but currently that results in inconsistent physical plans as the rule ProjectAwayColumns will wrongly eliminate this attribute from the execution plan.

[elasticsearchmachine]

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