Mark group-level labels in Label API as such.

What does this MR do?

The project label API returns the union of project labels and group labels. However, you cannot differentiate between them. This MR adds a new field is_project_label to indicate this label is a project label.

What are the relevant issue numbers?

Closes #56694 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer