### 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(source.estimatedRowSize(), equalTo(allFieldRowSize + Integer.BYTES * 2));
    }
    public void testQueryJustWithLimit() throws Exception {
        var optimized = optimizedPlan(physicalPlan("""
            from test
            | limit 3
            """));
        var limit = as(optimized, LimitExec.class);
        var exchange = asRemoteExchange(limit.child());
        var project = as(exchange.child(), ProjectExec.class);
        var extract = as(project.child(), FieldExtractExec.class);
        var source = source(extract.child());
        assertThat(source.estimatedRowSize(), equalTo(allFieldRowSize + Integer.BYTES));
    }
    public void testPushDownDisjunction() {
        var plan = physicalPlan("""
            from test
            | where emp_no == 10010 or emp_no == 10011
            """);
        var optimized = optimizedPlan(plan);
        var topLimit = as(optimized, LimitExec.class);
        var exchange = asRemoteExchange(topLimit.child());
        var project = as(exchange.child(), ProjectExec.class);
        var extractRest = as(project.child(), FieldExtractExec.class);
        var source = source(extractRest.child());
        assertThat(source.estimatedRowSize(), equalTo(allFieldRowSize + Integer.BYTES));
        var tqb = as(sv(source.query(), "emp_no"), TermsQueryBuilder.class);
        assertThat(tqb.fieldName(), is("emp_no"));
        assertThat(tqb.values(), is(List.of(10010, 10011)));
    }