Move all methods related to process management to its own module

As seen in !73049 (closed), we will re-use multiple methods related to managing processes. They are currently defined in sidekiq_cluster/sidekiq_cluster.rb (as they were only used there), but in the future we will use them in other places as well.

We can move all methods related to process management to its own module. Functionality won't change, only from where we call the methods will be a different module.