Describe the bug
Currently the firewall rule list is marked as ForceNew: true which causes appending a firewall rule trigger a destroy and recreate of all firewall rules. For example let's say the input policy of the data center is DROP and we allow ssh and proxmox API access in firewall rules. Adding a firewall rule would cause catastrophic issues in this scenario.

Expected behavior
I expect the provider to append the new rule to the existing firewall rules.