Add a postgres backend

[jwoertink]

Looks like the Rails version supports this

https://guides.rubyonrails.org/action_cable_overview.html#postgresql-adapter

https://github.com/rails/rails/blob/main/actioncable/lib/action_cable/subscription_adapter/postgresql.rb

[jwoertink]

Looks like Rails might go default with this....

rails/rails#50480

[jwoertink]

Adding on to this, it looks like Rails went with a different interface for this https://github.com/rails/solid_cable mainly because the PG NOTIFY system requires the data to be less than 8000 bytes so if you need to send larger payloads, you'd have to tell NOTIFY to send the record ID then do a separate fetch for it making things a lot more complex.