### 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
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 * 2));
        var eval = as(topNLocal.child(), EvalExec.class);
        var source = source(eval.child());
        
        assertThat(source.estimatedRowSize(), equalTo(Integer.BYTES * 2));
    }
    public void testProjectAfterTopN() throws Exception {
        var optimized = optimizedPlan(physicalPlan("""
            from test
            | sort emp_no
            | keep first_name
            | limit 2
            """));
        var topProject = as(optimized, ProjectExec.class);
        assertEquals(1, topProject.projections().size());
        assertEquals("first_name", topProject.projections().get(0).name());
        var topN = as(topProject.child(), TopNExec.class);
        var exchange = asRemoteExchange(topN.child());
        var project = as(exchange.child(), ProjectExec.class);
        List<String> projectionNames = project.projections().stream().map(NamedExpression::name).collect(Collectors.toList());
        assertTrue(projectionNames.containsAll(List.of("first_name", "emp_no")));
        var extract = as(project.child(), FieldExtractExec.class);
        var source = source(extract.child());
        assertThat(source.limit(), is(topN.limit()));
        assertThat(source.sorts(), is(sorts(topN.order())));