Add Firewall RuleSet support

[komer3]

📝 Description

What does this PR do and why is this change necessary?
Adding support for the new RuleSet api endpoints for firewalls

✔️ How to Test

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

How do I run the relevant unit/integration tests?
Added the relevant unit and integration test. You can run make test to run them.

📷 Preview

If applicable, include a screenshot or code snippet of this change. Otherwise, please remove this section.

[Copilot]

Pull Request Overview

This PR adds support for Firewall RuleSet functionality to the linodego client library. The implementation provides complete CRUD operations for managing firewall rule sets, which are collections of firewall rules that can be grouped and managed as a unit.

• Added new RuleSet type with support for inbound/outbound rule collections
• Implemented CRUD operations: List, Create, Get, Update, and Delete for firewall rule sets
• Enhanced FirewallRule type to support rule set references via a new Ruleset field

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
firewall_rulesets.go	Core implementation of RuleSet type and CRUD operations
firewall_rules.go	Enhanced FirewallRule with ruleset reference support and custom JSON marshaling
test/unit/firewall_rulesets_test.go	Comprehensive unit tests covering all RuleSet operations
test/unit/firewall_rules_test.go	Added JSON marshaling tests for FirewallRule
test/integration/firewall_rulesets_test.go	Integration test for complete CRUD workflow
test/integration/fixtures/TestFirewallRuleSets_CRUD.yaml	Test fixture data for integration tests

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[lgarber-akamai]

@komer3 Is there any chance you could re-point this PR at the tmp-firewall-ruleset branch? Once the feature is available in production we can put up a PR to merge that branch into main.

[lgarber-akamai]

@komer3 This looks great!

One small thing: could the events from section 7.8 be added to the EventAction const?

[lgarber-akamai]

Looks great! CI failures are unrelated and will be resolved in #843