Completely decouple config parsing from `Client` construction

[randomir]

Recently refactored dwave.cloud.config.load_config() is in a good shape, but we want to completely decouple config parsing from Client construction, so that config can be loaded independently and used outside of Client -- for example in dwave.cloud.api.client.DWaveAPIClient, its subclasses and by dwave.cloud.api.resources.*.

This should be combined with #426. And then #490, #440, #357 and other config improvements can follow.