Support multiple ASGs

[AmitBaranes]

Hi,
We would like to support multiple ASGs since some drone pipelines run under specific node groups. I couldn't find a way to implement that using the current solution, might be cool to have this as well.
Also, would be nice to scale based on pipeline name and not only based on the number of running in total.

for example -

DRONE_AGENT_AUTOSCALING_GROUP: asg-1 asg-2
DRONE_AGENT_MAX_BUILDS_TOTAL: 10 ( not mandatory )
DRONE_AGENT_MAX_BUILDS_PIPELINE: 
{
  "asg-1": {
    "regression": 3,
    "backend": 2
  },
  "asg-2": {
    "frontend": 5
  }
}

thanks for taking drone-autoscaler to the next level, much appreciated!