Support diff mode when using vyos_firewall_rules

This change adds support for [diff
mode](https://docs.ansible.com/ansible/latest/user_guide/playbooks_checkmode.html#using-diff-mode)
to `vyos_firewall_rules`.  This can aide someone into understanding what
will check when using [check
mode](https://docs.ansible.com/ansible/latest/user_guide/playbooks_checkmode.html#using-check-mode),
and can help someone know what has changed when running their play
(using just `--diff` without `--check`).

Author: sdwilsh

changelogs/fragments/diff-mode-firewall_rules.yml

@@ -0,0 +1,3 @@
+---
+major_changes:
+  - Support [diff mode](https://docs.ansible.com/ansible/latest/user_guide/playbooks_checkmode.html#using-diff-mode) in `vyos_firewall_rules`

plugins/module_utils/network/vyos/config/firewall_rules/firewall_rules.py

@@ -117,6 +117,17 @@ def execute_module(self):
             result["gathered"] = changed_firewall_rules_facts
 
         result["warnings"] = warnings
+
+        if self.state in self.ACTION_STATES and self._module._diff:
+            result["diff"] = {
+                "after": "\n".join(
+                    sorted(self.set_state(self._module.params["config"], {}))
+                ),
+                "before": "\n".join(
+                    sorted(self.set_state(existing_firewall_rules_facts, {}))
+                ),
+            }
+
         return result
 
     def set_config(self, existing_firewall_rules_facts):