### 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
| rename emp_no as x
            | keep x
            | eval y = x * 2""");
        var keep = as(plan, Project.class);
        var eval = as(keep.child(), Eval.class);
        assertThat(
            eval.fields(),
            contains(
                new Alias(
                    EMPTY,
                    "y",
                    new Mul(EMPTY, new FieldAttribute(EMPTY, "emp_no", mapping.get("emp_no")), new Literal(EMPTY, 2, INTEGER))
                )
            )
        );
    }
    public void testPushDownDissectPastProject() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | rename first_name as x
            | keep x
            | dissect x "%{y}"
            """);
        var keep = as(plan, Project.class);
        var dissect = as(keep.child(), Dissect.class);
        assertThat(dissect.extractedFields(), contains(new ReferenceAttribute(Source.EMPTY, "y", DataTypes.KEYWORD)));
    }
    public void testPushDownGrokPastProject() {
        LogicalPlan plan = optimizedPlan("""
            from test