Add syntax sugar for min_approvals

From @sshahcodes 

Can we abstract the lamda function so that users only need to write:
 
```python
(RequestPolicy.names == ("access-stage",))
& (RequestPolicy.names.min_approvals(1))
 ```

instead of:

```python
(RequestPolicy.names == ("access-stage",))
& (
RequestPolicy.names.for_each(
lambda x: RequestPolicy.approvals[x].len() > 1))
```
 