Added extra checkpoint data

[BloodAxe]

Allows to save additional metadata to checkpoint files:

runner.train(
        model=model,
        ....,
        extra_checkpoint_data={
            'class_names': ['Cat', 'Dogs']
        }
)

[Scitator]

let's just add checkpoint_data to state init, so we can use it also from Config API.
proposed way:

state_params:
    checkpoint_data:
        exp: "thin is my best experiments"
        date: "today"

This will also reduce such hasattr check.

[hexfaker]

Let's squash all small commits into one or two. Keep our history simple and clean!