Add Project Migration Labels

Mentions #339419 (closed)

⚠ this feature is behind bulk_import_projects feature flag that is currently off by default (and disabled on .com).

What does this MR do and why?

This MR adds Labels migration when using Group Migration that includes projects (https://docs.gitlab.com/ee/user/group/import/). It's identical to group labels migration, that is why LabelsPipeline has been updated from being groups only to being used in both Projects and Groups. This MR also updates a few BulkImports::Entity helper methods for convenience.

Note that currently project migration does not include repository import. It's being worked on in a separate MR.

Screenshots or screen recordings

projectmigrationlabels

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How to set up and validate locally

1. Feature.enable(:bulk_import)
2. Feature.enable(:bulk_import_projects)
3. Create a top level group and project in it
4. Create labels in newly created project
5. Go to /groups/new#import-group-pane page and enter instance url and access token (needs to be api scope)
6. Select newly created group and click Import
7. Wait for Group import to complete and verify imported project's labels

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.