Move decider_client_from_config to reddit_experiments

[julieahnt]

Summary

• This pr moves decider_client_from_config in reddit_decider that added from Decider #6 to the reddit_experiments package.
• Fix typing issues in reddit_decider

Usage

from reddit_experiments import decider_client_from_config
...
def my_field_extractor(request):                                                                   
    return {"foo": request.headers.get("Foo"), “bar”: “something”} 

def make_wsgi_app(app_config):
    ...
    baseplate.add_to_context(
        "decider",
        decider_client_from_config(
            app_config=app_config,
            event_logger=ExperimentLogger(),
            request_field_extractor=my_field_extractor,
        ),
    )