Support for object-oriented `WorkerSettings` classes

[chrisguidry]

In the current implementation, we're only looking at the __dict__ of the user's provided WorkerSettings class, which can be quite misleading if you're trying to get a large multi-queue system set up. The on_startup/on_shutdown hooks are similarly only allowed to be plain functions or @staticmethod, but not @classmethod because those aren't directly callable.

Example of things that could work naturally:

class BaseWorkerSettings:
    redis_settings = get_my_common_redis_settings()

    @classmethod
    async def on_startup(cls, context: dict[str, Any]):
        ...do some common initialization on all workers...

class FastLane(BaseWorkerSettings):
    queue_name = "arq:fast-lane"
    tasks = [task_a, task_b]


class SlowLane(BaseWorkerSettings):
    queue_name = "arq:slow-lane"
    tasks = [task_c, task_d]

    @classmethod
    async def on_startup(cls, context: dict[str, Any]):
        await super().on_startup(context))
        ...do some specialized initialization on just this worker...