Support running ActionCable on the same Puma process used for web requests

In #17589 (comment 295448279), we decided to start with a setup where ActionCable is run in a separate Puma process for isolation.

I think we should still support a setup where ActionCable is run on the same Puma server used to serve web requests so that we don't have another set of processes booting up the whole Rails application. This will save us some memory.

I think we should even consider making this the default setup for GDK / Omnibus.