Improve `show` for logger instances

[ablaom]

I think it would be better if a logger instance displays as you would construct it. That way the user can easily guess how to add keywords without looking up the docstring.

So, instead of

julia> MLFlowLogger("http://127.0.0.1:5000")
MLFlowLogger(MLFlow(
    baseuri = "http://127.0.0.1:5000", 
    apiversion = 2.0
), 1, "MLJ experiment", nothing)

we arrange

julia> MLFlowLogger("http://127.0.0.1:5000")
MLFlowLogger("http://127.0.0.1:5000",
    experiment_name="MLJ experiment",
    artifact_location=nothing,
)

This skips display of the apiversion, but I don't think that's a big deal. But we could do:

julia> MLFlowLogger("http://127.0.0.1:5000")
MLFlowLogger("http://127.0.0.1:5000",
    experiment_name="MLJ experiment",
    artifact_location=nothing,
) using MLflow version 2.0

@pebeto What do you think?