Add global CA/PVA address list NixOS option

[minijackson]

So that the end user only has to set it once for all services.

Something like this:

environment.epics = {
  ca_addr_list = [ "host1" "host2" ];
  ca_auto_addr_list = false;
};

And it would set:

{
  environment.variables.EPICS_CA_ADDR_LIST = "host1 host2";
  # and the same for ca_auto_addr_list

  services.archiver-appliance.settings.EPICS_CA_ADDR_LIST = [ "host1" "host2" ];
  services.phoebus-alarm-server.settings."org.phoebus.pv.ca/addr_list" = [ "host1" "host2" ];
  # etc.
}

cc @achoqt