### Description

Evals that introduce aliased can be simplified by extracting them into a project (and thus signaling there's no underlying processing).

The following eval:
`eval x = a + 1, y = x, z = y + 1, y = z, w = y + 1`
can be converted into:
`eval x = a + 1, z = a + 1 + 1, w = a + 1 + 1 | project x, z, z as y, w`

This helps fuse the project with other projections/aggs downstream.
x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LogicalPlanOptimizerTests.java
| eval x = emp_no
            | where x > 10
            | keep x
            """);
        var project = as(plan, EsqlProject.class);
        var limit = as(project.child(), Limit.class);
        var filter = as(limit.child(), Filter.class);
        var eval = as(filter.child(), Eval.class);
        assertThat(eval.fields(), hasSize(1));
        var alias = as(eval.fields().get(0), Alias.class);
        assertThat(Expressions.name(alias.child()), is("emp_no"));
        var source = as(eval.child(), EsRelation.class);
    }
    public void testEnrich() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | eval x = to_string(languages)
            | enrich languages_idx on x
            """);
        var enrich = as(plan, Enrich.class);
        assertTrue(enrich.policyName().resolved());
        assertThat(enrich.policyName().fold(), is(BytesRefs.toBytesRef("languages_idx")));
        var eval = as(enrich.child(), Eval.class);
        var limit = as(eval.child(), Limit.class);
        as(limit.child(), EsRelation.class);
    }
    public void testPushDownEnrichPastProject() {
        LogicalPlan plan = optimizedPlan("""
            from test
            | eval a = to_string(languages)