### 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/PhysicalPlanOptimizerTests.java
assertThat(aggregate.estimatedRowSize(), equalTo(Long.BYTES));
        var extract = as(aggregate.child(), FieldExtractExec.class);
        assertThat(names(extract.attributesToExtract()), contains("emp_no"));
    }
    public void testQueryWithNull() {
        var plan = physicalPlan("""
            from test
            | eval nullsum = emp_no + null
            | sort emp_no
            | limit 1
            """);
        var optimized = optimizedPlan(plan);
        var topN = as(optimized, TopNExec.class);
        
        assertThat(topN.estimatedRowSize(), equalTo(0));
        var exchange = asRemoteExchange(topN.child());
        var project = as(exchange.child(), ProjectExec.class);
        var extract = as(project.child(), FieldExtractExec.class);
        var topNLocal = as(extract.child(), TopNExec.class);
        
        assertThat(topNLocal.estimatedRowSize(), equalTo(allFieldRowSize + Integer.BYTES));
        var extractForEval = as(topNLocal.child(), FieldExtractExec.class);
        var eval = as(extractForEval.child(), EvalExec.class);
        var source = source(eval.child());
        
        assertThat(source.estimatedRowSize(), equalTo(Integer.BYTES * 2 + Integer.BYTES));
    }
    public void testPushAndInequalitiesFilter() {
        var plan = physicalPlan("""