Add accessor method to extract the "client" from an MLFlowLogger object

[ablaom]

As discussed on a call.

I know we have been calling this client but I wonder if this is really the correct word, since it is on a server most (all?) of the time. I see that MLFlowClient.jl documentation calls an instance of MLFlow an "MLFlow API service", so maybe "service" is a better work for this.

[pebeto]

You're completely right. service is a better name for this kind of instance.