### Description

In cases where the same agg is used, instead of computing it multiple times, make another reference to it:
```
stats a = min(x), b = min(x) by z
```

should become
```
stats a = min(x) by z
eval b = a
keep a,b,z
```

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizer.java
@Override
        protected LogicalPlan rule(Aggregate agg) {
            OrderBy order = findPullableOrderBy(agg.child());
            LogicalPlan p = agg;
            if (order != null) {
                p = agg.transformDown(OrderBy.class, o -> o == order ? order.child() : o);
            }
            return p;
        }
        private static OrderBy findPullableOrderBy(LogicalPlan plan) {
            OrderBy pullable = null;
            if (plan instanceof OrderBy o) {
                pullable = o;
            } else if (plan instanceof Eval
                || plan instanceof Filter
                || plan instanceof Project
                || plan instanceof RegexExtract
                || plan instanceof Enrich) {
                    pullable = findPullableOrderBy(((UnaryPlan) plan).child());
                }
            return pullable;
        }
    }
    static class PruneRedundantSortClauses extends OptimizerRules.OptimizerRule<OrderBy> {