Make lower-level utilities available to third-party frameworks

**Current Problem**
I want to use some of the the low-level utilities provided in `dwave-cloud-client` directly from a third-party sampler.

For example, I'd like to validate that my problem is compatible with a device's graph. This functionality is available in the `DWaveSampler`'s underlying [`Client`](https://github.com/dwavesystems/dwave-cloud-client/blob/master/dwave/cloud/solver.py#L975), but not directly from the sampler; I have to access the low-level `Client`. However, a third-party sampler does not have an underlying `Client`, so it becomes impossible to use this feature.

**Proposed Solution**
Expose useful lower-level utilities through the `Sampler` interface.