new: add support listing nodebalancer firewalls (#436)

## 📝 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 ARGS='-run TestNodeBalancerFirewalls_List' fixtures
```

nodebalancer_firewalls.go

@@ -0,0 +1,41 @@
+package linodego
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/go-resty/resty/v2"
+)
+
+// NodeBalancerFirewallsPagedResponse represents a Linode API response for listing of Cloud Firewalls
+type NodeBalancerFirewallsPagedResponse struct {
+	*PageOptions
+	Data []Firewall `json:"data"`
+}
+
+func (NodeBalancerFirewallsPagedResponse) endpoint(ids ...any) string {
+	id := ids[0].(int)
+	return fmt.Sprintf("nodebalancers/%d/firewalls", id)
+}
+
+func (resp *NodeBalancerFirewallsPagedResponse) castResult(r *resty.Request, e string) (int, int, error) {
+	res, err := coupleAPIErrors(r.SetResult(NodeBalancerFirewallsPagedResponse{}).Get(e))
+	if err != nil {
+		return 0, 0, err
+	}
+	castedRes := res.Result().(*NodeBalancerFirewallsPagedResponse)
+	resp.Data = append(resp.Data, castedRes.Data...)
+	return castedRes.Pages, castedRes.Results, nil
+}
+
+// ListNodeBalancerFirewalls returns a paginated list of Cloud Firewalls for nodebalancerID
+func (c *Client) ListNodeBalancerFirewalls(ctx context.Context, nodebalancerID int, opts *ListOptions) ([]Firewall, error) {
+	response := NodeBalancerFirewallsPagedResponse{}
+
+	err := c.listHelper(ctx, &response, opts, nodebalancerID)
+	if err != nil {
+		return nil, err
+	}
+
+	return response.Data, nil
+}