ESQL: Enforce uniquely named attributes in query plans

[alex-spies]

Relates #110490

In contrast to other commands/query plan nodes, ROW a= 1, a = 2 creates 2 attributes with the same name a; this is a problem because one cannot refer to any of the as as there's no way to disambiguate.

Let's fix this and ensure all our query plans consistently handle multiple created attributes with the same name, like EVAL x = 1, x = 2 (or dissect, enrich, ...)

[elasticsearchmachine]

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