### 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/LogicalPlanOptimizerTests.java
var keep = as(plan, Project.class);
        var limit = as(keep.child(), Limit.class);
        var filter = as(limit.child(), Filter.class);
        var attr = filter.condition().collect(Attribute.class::isInstance).stream().findFirst().get();
        assertThat(as(attr, ReferenceAttribute.class).name(), is("y"));
        var dissect = as(filter.child(), Dissect.class);
        as(dissect.child(), EsRelation.class);
    }
    public void testPushDownFilterPastProjectUsingGrok() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | grok first_name "%{WORD:y}"
            | rename y as x
            | keep x
            | where x == "foo"
            """);
        var keep = as(plan, Project.class);
        var limit = as(keep.child(), Limit.class);
        var filter = as(limit.child(), Filter.class);
        var attr = filter.condition().collect(Attribute.class::isInstance).stream().findFirst().get();
        assertThat(as(attr, ReferenceAttribute.class).name(), is("y"));
        var grok = as(filter.child(), Grok.class);
        as(grok.child(), EsRelation.class);
    }
    public void testPushDownLimitPastEval() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | eval x = emp_no + 100
            | limit 10""");
        var eval = as(plan, Eval.class);