[ENH] - Add kubernetes horizontal autoscaler for conda-store workers based on queue depth

[dcmcand]

Feature description

Currently conda-store is set to allow 4 simultaneous builds at once. This is a bottleneck once multiple environments start getting built at once and presents a scaling challenge. If we set the simultaneous builds to 1 and autoscale based on queue depth then we should be able to handle scaling far more gracefully

Value and/or benefit

Having the conda-store workers autoscale based on queue depth will allow larger orgs to take advantage of Nebari without hitting scale bottlenecks.

Anything else?

https://learnk8s.io/scaling-celery-rabbitmq-kubernetes