Add firewall_id field to NodePool

[rammanoj]

📝 Description

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

This PR adds the firewall_id field to lkenodepool. LKE-E has support to create a nodepool with existing firewall / update the firewall of existing nodepool. This PR adds the firewall_id support to linodego.

✔️ How to Test

What are the steps to reproduce the issue or verify the changes?

• Create 2 firewalls (firewallA & firewallB)
• Create a enterprise cluster with the following payload.

	clusterReq := linodego.LKEClusterCreateOptions{
		Label:      "enterprise-cluster",
		Region:     "us-ord",
		K8sVersion: "v1.31.9+lke7",
		Tier:       "enterprise",
		// Enterprise flag not public; contact Linode to enable Enterprise tier
		NodePools: []linodego.LKENodePoolCreateOptions{{
			Type:       "g6-standard-4",
			Count:      3,
			FirewallID: linodego.Pointer(<firewallA-id>),
		}},
	}

• Ensure the nodes joined the cluster and use the firewall created in step 1.
• Now, change the firewall using the below API and ensure that it returns a 200.

	_, err := client.UpdateLKENodePool(context.Background(), <cluster-id>, <pool-id>, linodego.LKENodePoolUpdateOptions{
		FirewallID: linodego.Pointer(<firewallB-id>),
	})
	if err != nil {
		log.Fatalf("Error updating node pool: %v", err)
	}

How do I run the relevant unit/integration tests?

Run make test-unit TEST_ARGS='-run TestLKENodePool'

📷 Preview

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