[FEATURE REQUEST] Task aggregation config by group

Is your feature request related to a problem? Please describe. I have a situation where I want to process multiple group tasks, but there are some groups that need to have a different configuration than the others.

For example, set the default global config

• GroupMaxDelay 10 minutes
• GroupGracePeriod 2 * time.Minute
• GroupMaxSize 20

For group A, it's okay to use the config from the default global config. but for Group B, I need different configs, for example GroupGracePeriod set to 1 minute or GroupMaxSize set to 50

Describe the solution you'd like Support config by group name, maybe something like this

  asynq.Config{
     Queues: map[string]int{"notifications": 1},
     GroupAggregator:  asynq.GroupAggregatorFunc(aggregate),
     GroupMaxDelay:    10 * time.Minute,
     GroupGracePeriod: 2 * time.Minute,
     GroupMaxSize:     20,
     GroupConfigs : map[string]asynq.GroupConfig{
           GroupMaxDelay:    5 * time.Minute,
           GroupGracePeriod: 1 * time.Minute,
           GroupMaxSize:     30,
    }
 },