ESQL: ENRICH attribute handling inconsistent with EVAL, GROK, ...

[alex-spies]

For ENRICH with a WITH statement, the attributes it creates are handled differently from EVAL and GROK/DISSECT.

E.g. a query with... | ENRICH languages_policy ON a WITH name = language_name has an Enrich logical/physical plan, and the Expression for the language_name attribute is an Alias called name.

Weirdly, we need to handle the unaliased language_name ReferenceAttribute directly in PhysicalPlanOptimizer.ProjectAwayColumns. A similar hack is currently required to make the DependencyCheck work on LogicalPlans.

This should be refactored so that the attribute handling is analogous to EVAL and GROK/DISSECT.

[elasticsearchmachine]

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