Transfer group labels

What does this MR do and why?

Resolves #354890 (closed)

The MR updates Groups::TransferService to use the existing Labels::TransferService to transfer labels for each project under the transferred group. #354890 (closed) describes why this is necessary. To provide a short summary, Groups::TransferService does not currently copy/re-assign group labels. For example, if you were to transfer the GitLab project from the GitLab.org group to the GitLab-com group, the issues that may have the labels from the GitLab.org group would still have them associated.

The change is shipped behind a feature flag for de-risking.

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:group_labels_transfer)

2. Create the following domain objects.

• Create a group named "Root group". As the name indicated, it should be a root group.

  • Create a group label belonging to "Root group". Name it "orange".

• Create a group named "Sub group". The group should belong to "Root group"

  • Create a group label belonging to "Sub group". Name it "blue".

• Create a project under "Sub group"

  • Create a project label belonging to the created project. Name it "red".

  • Create an issue. Assign ALL labels (orange, blue, and red) to the issue.

• Create a group named "Root group 2". As the name indicated, it should be a root group.

3. Transfer "Sub group" to "Root group 2".

Confirm that the project under "Sub group" has a new label titled "orange". Previously the labels from a parent group (the parent of the transferred group) were not copied.

Before transfer	After transfer

More cases are covered in the updated rspec file.