Support external CUPS server

[horcsinbalint]

It is sometimes better to have a CUPS server that is located on a different server. (Like when the CUPS server is running nearby the printer on a different floor.) There is currently no supported method of setting the IP address and port of the CUPS server for commands lp and lpstat in mars. It could be beneficial to add an additional field in .env which allows arbitrary command line arguments to be inserted in front of the -d and -W directive respectively.

[kdmnk]

Added to #5

[horcsinbalint]

My original description did not fully show the importance of supporting external CUPS servers.

We have deployed a Raspberry PI next to the printer and connected the printer and RPI with a USB cable. The CUPS server is running on the RPI, so Mars needs to connect to that CUPS server. We have made a secure connection between the RPI and Mars's PHP FPM (with something similar to a VPN).

The changes had two main goals. One of them was to make our systems more resilient against attacks. A less important one was that from now on printers without networking capabilities would be also a great option for the dormitory.

Similarly to #364, the changes that were done in the deployment branch seem to allow these settings. And I also plan to make the proper changes on the upstream development branch. Unfortunately, I did not have enough time to do it yet.

Therefore, I would like to reopen this issue for a potential second round of evaluation.