new: add support for listing nodebalancer firewalls (#356)

## 📝 Description

**What does this PR do and why is this change necessary?**

Add support for `nodebalancers/{nb_id}/firewalls` endpoint

## ✔️ How to Test

**How do I run the relevant unit/integration tests?**

```bash
make testunit
```

Author: jriddle-linode

linode_api4/objects/nodebalancer.py

@@ -9,7 +9,7 @@
     Property,
     Region,
 )
-from linode_api4.objects.networking import IPAddress
+from linode_api4.objects.networking import Firewall, IPAddress
 
 
 class NodeBalancerNode(DerivedBase):
@@ -303,3 +303,21 @@ def statistics(self):
                 "Unexpected response generating stats!", json=result
             )
         return MappedObject(**result)
+
+    def firewalls(self):
+        """
+        View Firewall information for Firewalls associated with this NodeBalancer.
+
+        API Documentation: https://www.linode.com/docs/api/nodebalancers/#nodebalancer-firewalls-list
+
+        :returns: A List of Firewalls of the Linode NodeBalancer.
+        :rtype: List[Firewall]
+        """
+        result = self._client.get(
+            "{}/firewalls".format(NodeBalancer.api_endpoint), model=self
+        )
+
+        return [
+            Firewall(self._client, firewall["id"])
+            for firewall in result["data"]
+        ]

test/fixtures/nodebalancers_12345_firewalls.json

@@ -0,0 +1,56 @@
+{
+  "data": [
+    {
+      "created": "2018-01-01T00:01:01",
+      "id": 123,
+      "label": "firewall123",
+      "rules": {
+        "inbound": [
+          {
+            "action": "ACCEPT",
+            "addresses": {
+              "ipv4": [
+                "192.0.2.0/24"
+              ],
+              "ipv6": [
+                "2001:DB8::/32"
+              ]
+            },
+            "description": "An example firewall rule description.",
+            "label": "firewallrule123",
+            "ports": "22-24, 80, 443",
+            "protocol": "TCP"
+          }
+        ],
+        "inbound_policy": "DROP",
+        "outbound": [
+          {
+            "action": "ACCEPT",
+            "addresses": {
+              "ipv4": [
+                "192.0.2.0/24"
+              ],
+              "ipv6": [
+                "2001:DB8::/32"
+              ]
+            },
+            "description": "An example firewall rule description.",
+            "label": "firewallrule123",
+            "ports": "22-24, 80, 443",
+            "protocol": "TCP"
+          }
+        ],
+        "outbound_policy": "DROP"
+      },
+      "status": "enabled",
+      "tags": [
+        "example tag",
+        "another example"
+      ],
+      "updated": "2018-01-02T00:01:01"
+    }
+  ],
+  "page": 1,
+  "pages": 1,
+  "results": 1
+}

test/unit/objects/nodebalancers_test.py

@@ -193,3 +193,15 @@ def test_statistics(self):
                 "linode.com - balancer12345 (12345) - day (5 min avg)",
             )
             self.assertEqual(m.call_url, statistics_url)
+
+    def test_firewalls(self):
+        """
+        Test that you can get the firewalls for the requested NodeBalancer.
+        """
+        nb = NodeBalancer(self.client, 12345)
+        firewalls_url = "/nodebalancers/12345/firewalls"
+
+        with self.mock_get(firewalls_url) as m:
+            result = nb.firewalls()
+            self.assertEqual(m.call_url, firewalls_url)
+            self.assertEqual(len(result), 1)