TPT-4320: Update FirewallID to use single pointer in LinodeInterfaceCreateOptions

[zliang-akamai]

Only -1 can unset firewall ID now when default firewall setting is in-place for Linode interface. Update it to a single pointer because we no longer need the double pointer to explicitly express null in API requests.

[Copilot]

Pull request overview

Updates Linode Interface creation options to reflect the API change where firewall_id no longer needs to support explicit null (using -1 to unset instead), simplifying the SDK field type.

Changes:

• Changed LinodeInterfaceCreateOptions.FirewallID from **int to *int.
• Updated unit and integration tests to use linodego.Pointer(...) instead of linodego.DoublePointer(...).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
interfaces.go	Simplifies FirewallID type on LinodeInterfaceCreateOptions to a single pointer.
test/unit/interface_test.go	Updates interface creation unit test to match the new FirewallID type.
test/integration/instance_interfaces_test.go	Updates integration test assignments to match the new FirewallID type.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[ezilber-akamai]

Should we leave a comment notifying users of the breaking change? We can include it in the release notes but a comment with a TODO to remove in ~6 months or so could be nice too.

[zliang-akamai]

@ezilber-akamai I think we are fine to just make the breaking change as the project is still in LA/beta, not stable status.

And the breaking change label on this PR will be converted to a breaking change notice into the generated release note.