### 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
Filter expected = new Filter(
            EMPTY,
            new Aggregate(
                EMPTY,
                new Filter(EMPTY, relation, new And(EMPTY, conditionA, conditionB)),
                singletonList(getFieldAttribute("b")),
                emptyList()
            ),
            aggregateCondition
        );
        assertEquals(expected, new LogicalPlanOptimizer.PushDownAndCombineFilters().apply(fb));
    }
    public void testSelectivelyPushDownFilterPastRefAgg() {
        
        LogicalPlan plan = optimizedPlan("""
            from test
            | where emp_no > 1
            | stats x = count(1) by emp_no
            | 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 GreaterThan);
        var gt = (GreaterThan) filter.condition();
        assertTrue(gt.left() instanceof ReferenceAttribute);
        var refAttr = (ReferenceAttribute) gt.left();
        assertEquals("x", refAttr.name());
        assertEquals(L(7), gt.right());
        var agg = as(filter.child(), Aggregate.class);