Adding support for PrefixList APIs

[komer3]

📝 Description

What does this PR do and why is this change necessary?
Adding support for listing and getting specific PrefixList APIs

✔️ 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 relevant unit tests (no integration since API is not currently available). Just run make test to verfiy

📷 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 PrefixList APIs by implementing functionality to list and retrieve specific prefix lists from the Linode API.

• Introduces a new PrefixList struct with all necessary fields and custom JSON unmarshaling for timestamp handling
• Adds ListPrefixLists and GetPrefixList client methods for API interaction
• Provides comprehensive unit tests covering list, get, and JSON unmarshaling operations

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
prefixlists.go	Implements PrefixList struct, custom JSON unmarshaling, and client methods for listing and getting prefix lists
test/unit/prefixlists_test.go	Adds comprehensive unit tests for PrefixList operations including list, get, and JSON unmarshaling scenarios

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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.