### 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/test/java/org/elasticsearch/xpack/esql/optimizer/LocalPhysicalPlanOptimizerTests.java
import org.elasticsearch.test.ESTestCase;
import org.elasticsearch.xpack.core.enrich.EnrichPolicy;
import org.elasticsearch.xpack.esql.EsqlTestUtils;
import org.elasticsearch.xpack.esql.EsqlTestUtils.TestSearchStats;
import org.elasticsearch.xpack.esql.analysis.Analyzer;
import org.elasticsearch.xpack.esql.analysis.AnalyzerContext;
import org.elasticsearch.xpack.esql.analysis.EnrichResolution;
import org.elasticsearch.xpack.esql.analysis.Verifier;
import org.elasticsearch.xpack.esql.enrich.EnrichPolicyResolution;
import org.elasticsearch.xpack.esql.expression.function.EsqlFunctionRegistry;
import org.elasticsearch.xpack.esql.parser.EsqlParser;
import org.elasticsearch.xpack.esql.plan.physical.AggregateExec;
import org.elasticsearch.xpack.esql.plan.physical.EsQueryExec;
import org.elasticsearch.xpack.esql.plan.physical.EsStatsQueryExec;
import org.elasticsearch.xpack.esql.plan.physical.EsStatsQueryExec.Stat;
import org.elasticsearch.xpack.esql.plan.physical.EstimatesRowSize;
import org.elasticsearch.xpack.esql.plan.physical.ExchangeExec;
import org.elasticsearch.xpack.esql.plan.physical.LimitExec;
import org.elasticsearch.xpack.esql.plan.physical.LocalSourceExec;
import org.elasticsearch.xpack.esql.plan.physical.PhysicalPlan;
import org.elasticsearch.xpack.esql.planner.FilterTests;
import org.elasticsearch.xpack.esql.planner.Mapper;
import org.elasticsearch.xpack.esql.planner.PlannerUtils;
import org.elasticsearch.xpack.esql.plugin.QueryPragmas;
import org.elasticsearch.xpack.esql.session.EsqlConfiguration;
import org.elasticsearch.xpack.esql.stats.Metrics;
import org.elasticsearch.xpack.esql.stats.SearchStats;
import org.elasticsearch.xpack.esql.type.EsqlDataTypes;
import org.elasticsearch.xpack.ql.expression.Expressions;
import org.elasticsearch.xpack.ql.expression.function.FunctionRegistry;