Add the ability to filter out infeasible samples when calling the LeapHybridCQMSampler

Currently this can be done with
```python
sampleset = LeapHybridCQMSampler().sample_cqm(cqm)
feasible_sampleset = sampleset.filter(lambda d: d.is_feasible)
```
see https://github.com/dwavesystems/dimod/blob/ec9c3b274ff4104c00fe702d680b320f0bffca25/dimod/sampleset.py#L1371

It would be relatively straightforward to add a `filter_feasible` (or similar) keyword argument to the `LeapHybridCQMSampler.sample_cqm()` method. Something like
```python
feasible_sampleset = LeapHybridCQMSampler().sample_cqm(cqm, filter_feasible=True)
```

Some issues:
* Need to decide what to do in the case that there are no feasible solutions. Probably return an empty sample set, but could consider raising an exception.
* Would need to make `SampleSet.filter()` non-blocking, or handle this server-side. I don't think there is much performance benefit from doing it server-side though.

I personally feel like the existing syntax is explicit and unsurprising at the cost of one additional line of code, but perhaps some syntactic sugar here would be appreciated by users.