Utility function or a Composite for estimating "optimal" chain_strength

[`EmbeddingComposite`](https://github.com/dwavesystems/dwave-system/blob/9c447fe73589e9ee4fee713a8b6d6eed1ff63366/dwave/system/composites/embedding.py#L46) et al. currently use a fixed default value for `chain_strength` (`1.0`), regardless of actual problem bias/coupling magnitudes.

It would be useful to provide a utility for estimating a better `chain_strength`, based on actual problem being embedded. Wrapping that utility in a sampling composite would be very helpful in a use case of multiple/auto embedding calls, typical in a hybrid workflow.