feat: support link variant for migration client

[xrutayisire]

Resolves: DT-2476: AAUser I can use the Migration API client to migrate documents that contain links with variants

Description

• Remove some TODO that we forgot before
• Add support for link variant in migration api client

Checklist

• A comprehensive Linear ticket, providing sufficient context and details to facilitate the review of the PR, is linked to the PR.
• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

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