[FEATURE REQUEST] Customize Queue Group Strategy.

[dusty-cjh]

Hi master, thanks for your effort first.

Is your feature request related to a problem? Please describe.
Redis Memory is crucial to every user.
In our scenario, we queue huge among of tasks to redis, and then group and process them.
Here is the hiccup, we want to monitor the existing grouped tasks' storage usage to determine whether or not to handle the tasks. But such rules don't exist.

Describe the solution you'd like
The broker argument is explicitly set to a fixed value: RDB, If you can expose this interface for us, we'll appreciate it a lot.

Additional context
And by the way, I'm not professional in using asynq as you, so if there already have existing solution, or anywhere I'm wrong in description, please tell me, thanks.