### 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
CombineEvals() {
            super(TransformDirection.UP);
        }
        @Override
        protected LogicalPlan rule(Eval eval) {
            LogicalPlan plan = eval;
            if (eval.child() instanceof Eval subEval) {
                plan = new Eval(eval.source(), subEval.child(), CollectionUtils.combine(subEval.fields(), eval.fields()));
            }
            return plan;
        }
    }
    
    
    
    
    static class PropagateEvalFoldables extends Rule<LogicalPlan, LogicalPlan> {