### 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
*/
    public void testPruningDuplicateEvals() {
        var plan = plan("""
              from test
            | eval x = emp_no, x = salary
            | eval y = salary
            | eval y = emp_no
            | keep x, y
            """);
        var project = as(plan, Project.class);
        var projections = project.projections();
        assertThat(Expressions.names(projections), contains("x", "y"));
        var child = aliased(projections.get(0), FieldAttribute.class);
        assertThat(child.name(), is("salary"));
        child = aliased(projections.get(1), FieldAttribute.class);
        assertThat(child.name(), is("emp_no"));
        var limit = as(project.child(), Limit.class);
        var source = as(limit.child(), EsRelation.class);
    }
    /**
     * Expects
     * Limit[500[INTEGER]]
     * \_Aggregate[[],[COUNT(salary{f}#24) AS cx, COUNT(emp_no{f}#19) AS cy]]
     *   \_EsRelation[test][_meta_field{f}#25, emp_no{f}#19, first_name{f}#20, ..]
     */
    public void testPruneEvalAliasOnAggUngrouped() {
        var plan = plan("""
              from test
            | eval x = emp_no, x = salary
            | eval y = salary