Use Loadbalancer ID as marker

### Summary

Have Loadbalancer service use the loadbalancer ID as a marker instead of the loadbalancer name.

### Approach

* Create `\OpenCloud\LoadBalancer\Collection\LoadBalancerIterator` class.
* Override the `collection()` method from the `AbstractService` method in `\OpenCloud\LoadBalancer\Service` class.

This is done in a similar manner to the `DnsIterator` class.

Fixes #502.

### Risk
- [x] Low

PR is this risk because we're keeping the existing `PaginatedIterator` functionality, and only changing the defaults.

### Tests
- [ ] Unit

**Smoke Test Instructions**
Calling `\OpenCloud\LoadBalancer\Service::loadBalancerList()->constructNextUrl()` should result in a URL with the loadbalancer ID being used as a marker instead of the laodbalancer name.