### Description

Evals that introduce aliased can be simplified by extracting them into a project (and thus signaling there's no underlying processing).

The following eval:
`eval x = a + 1, y = x, z = y + 1, y = z, w = y + 1`
can be converted into:
`eval x = a + 1, z = a + 1 + 1, w = a + 1 + 1 | project x, z, z as y, w`

This helps fuse the project with other projections/aggs downstream.
x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizer.java
@Override
        protected LogicalPlan rule(RegexExtract re) {
            LogicalPlan child = re.child();
            if (child instanceof OrderBy orderBy) {
                return orderBy.replaceChild(re.replaceChild(orderBy.child()));
            } else if (child instanceof Project) {
                var projectWithChild = pushDownPastProject(re);
                return projectWithChild.withProjections(mergeOutputExpressions(re.extractedFields(), projectWithChild.projections()));
            }
            return re;
        }
    }
    
    protected static class PushDownEnrich extends OptimizerRules.OptimizerRule<Enrich> {
        @Override
        protected LogicalPlan rule(Enrich re) {
            LogicalPlan child = re.child();
            if (child instanceof OrderBy orderBy) {
                return orderBy.replaceChild(re.replaceChild(orderBy.child()));
            } else if (child instanceof Project) {
                var projectWithChild = pushDownPastProject(re);
                var attrs = asAttributes(re.enrichFields());
                return projectWithChild.withProjections(mergeOutputExpressions(attrs, projectWithChild.projections()));
            }
            return re;
        }
    }
    protected static class PushDownAndCombineOrderBy extends OptimizerRules.OptimizerRule<OrderBy> {