Support direct class setting

[masanorihirano]

Currently, classes are set by a string in config JSON/python dict.

When python dict is used for setting, we should support Type direction in the setting.

For example,

class UserDefinedAgent(Agent):
    pass

setting = {
    ....
    "Agents": {
        "class": UserDefinedAgent,
        ....
    }
}

runner = SequentialRunner(
    settings=config,
    ...
)

...

if it is possible, class_registration to the runner is not necessary.