Refactor: Move optional MlosCoreOptimizer config elements into a "config" section as others do

[bpkroth]

Nearly all other configs follow the form:

{
    "class": "class.name",
    "config": {
        ...
    }
}

The optimizer json schema intermingles a few things though:

{
    "class": "mlos_bench.optimizers.MlosCoreOptimizer",
    "config": {
        "optimizer_type": "SMAC",
        "n_random_init": 10 // <-- this isn't valid if optimizer_type is "FLAML"
    }
}

Instead it should really be:

{
    "class": "mlos_bench.optimizers.MlosCoreOptimizer",
    "config": {
        "optimizer_type": "SMAC",
       "optimizer_args": {
            "n_random_init": 10
        }
    }
}

Which is what we do for space_adapter_args for instance.

[bpkroth]

NOTE: this will be a breaking change and will need a new version cut for it