[FEATURE REQUEST] GroupMaxSize per queue

[liron-navon]

Is your feature request related to a problem? Please describe.
I work with multiple queues with one asynq server, I have multiple queues and multiple groups, we can pass "GroupMaxSize" to the asynq server, but this applies to ALL the groups, I'd like to be able to set a max size per group or per queue, since some groups might make sense to have 500 max size and others 100 max size.

Describe the solution you'd like
I'd suggest an interface like so where we can follow the "Queues" options style by setting a "Groups"
and each group can have its own options, or if it is too complicated just pass an int for the max size?

srv := asynq.NewServerFromRedisClient(
	tdb,
	asynq.Config{
		Concurrency:      10,
		GroupAggregator:  asynq.GroupAggregatorFunc(MyAggregate),
		GroupMaxDelay:    2 * time.Minute, // used as default
		GroupGracePeriod: 1 * time.Minute,  // used as default
		GroupMaxSize:     100,  // used as default
		Queues: map[string]int{
			"queue_A":         6,
			"queue_B":         2,
			"queue_C":         2
		},
		// OPTION 1
		Groups: map[string]GroupOptions{
			"queue_A": GroupOptions{
				GroupMaxDelay:    10 * time.Minute, 
				GroupGracePeriod: 2 * time.Minute,
				GroupMaxSize:     1000,
			},
			"queue_B": GroupOptions{
				GroupMaxSize:     300,
			},
		}
		// OPTION 2
		Groups: map[string]int{
			"queue_A": 1000,
			"queue_B": 300,
		}
	},
)

type GroupOptions struct {
	GroupMaxDelay      time.Duration
	GroupGracePeriod  time.Duration
	GroupMaxSize         int
}

Describe alternatives you've considered
alternative solutions are to create multiple instances of the asynq server, but that have performance panelties and makes the code more complex.

Using a single MaxSize for all the groups, but that reduces my options to optimize bulk insertions and bulk processing.

[faleksic]

I'd like to take a stab at this if this issue is open and something you want to support.
Basically I would follow the suggested config structure in this ticket and made groups optional in order to keep it backward compatible. I am open to other suggestions if you have some