### 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
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules.PruneLiteralsInOrderBy;
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules.SetAsOptimized;
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules.SimplifyComparisonsArithmetics;
import org.elasticsearch.xpack.ql.plan.logical.Aggregate;
import org.elasticsearch.xpack.ql.plan.logical.Filter;
import org.elasticsearch.xpack.ql.plan.logical.Limit;
import org.elasticsearch.xpack.ql.plan.logical.LogicalPlan;
import org.elasticsearch.xpack.ql.plan.logical.OrderBy;
import org.elasticsearch.xpack.ql.plan.logical.Project;
import org.elasticsearch.xpack.ql.plan.logical.UnaryPlan;
import org.elasticsearch.xpack.ql.rule.Rule;
import org.elasticsearch.xpack.ql.rule.RuleExecutor;
import org.elasticsearch.xpack.ql.util.CollectionUtils;
import org.elasticsearch.xpack.ql.util.Holder;
import java.time.ZoneId;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.function.Predicate;
import static java.util.Arrays.asList;
import static org.elasticsearch.xpack.esql.expression.NamedExpressions.mergeOutputExpressions;
import static org.elasticsearch.xpack.ql.expression.Expressions.asAttributes;
import static org.elasticsearch.xpack.ql.optimizer.OptimizerRules.FoldNull;
import static org.elasticsearch.xpack.ql.optimizer.OptimizerRules.PropagateEquals;
import static org.elasticsearch.xpack.ql.optimizer.OptimizerRules.PropagateNullable;
import static org.elasticsearch.xpack.ql.optimizer.OptimizerRules.TransformDirection;
public class LogicalPlanOptimizer extends RuleExecutor<LogicalPlan> {