### 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
from test
            | stats x = count(1) by emp_no
            | where (emp_no < 3 or x > 9) and emp_no > 0""");
        var limit = as(plan, Limit.class);
        var filter = as(limit.child(), Filter.class);
        assertTrue(filter.condition() instanceof Or);
        var or = (Or) filter.condition();
        assertTrue(or.left() instanceof LessThan);
        assertTrue(or.right() instanceof GreaterThan);
        var stats = as(filter.child(), Aggregate.class);
        filter = as(stats.child(), Filter.class);
        assertTrue(filter.condition() instanceof GreaterThan);
        var gt = (GreaterThan) filter.condition();
        assertTrue(gt.left() instanceof FieldAttribute);
        assertEquals("emp_no", ((FieldAttribute) gt.left()).name());
        assertEquals(L(0), gt.right());
        assertTrue(filter.child() instanceof EsRelation);
    }
    public void testSelectivelyPushDownFilterPastEval() {
        
        LogicalPlan plan = optimizedPlan("""
            from test
            | where emp_no > 1
            | eval x = emp_no + 1
            | where x + 2 < 9
            | where emp_no < 3""");
        var limit = as(plan, Limit.class);
        var filter = as(limit.child(), Filter.class);
        assertTrue(filter.condition() instanceof LessThan);
        var lt = (LessThan) filter.condition();