[proposal] Add support for throttling per route

API Gateway has various forms of API throttling. This proposal adds support for throttling at the routing level. This allows you to specify a rate and burst limit per @app.route.

Similar to CORS and authorizers, there will be a new param, throttle, that takes an instance of ThrottleConfig:

@app.route('/', throttle=ThrottleConfig(rate=10, burst=15))
def index(): pass

ThrottleConfig accepts two arguments, rate and burst:

class ThrottleConfig(object):
    def __init__(self, rate: int, burst: int): pass

The rate and burst params map to the corresponding concepts described in the API Gateway throttling docs