Add dependency_list_export model

What does this MR do and why?

Add dependency_list_export model which will be used by the export feature.

Currently dependency list exporter isn't persisted and its endpoint is shared with regular fetching with the purpose of providing data to be populated by the frontend code.

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/383052

The DB table was added as part of !104361 (merged)

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