Add merge request pipeline flag to pipeline entity

What does this MR do?

It turned out there is no way to detect attached merge request pipelines on frontend side (See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25848#note_148132516). This MR adds the flag in PipelineEntity.

This MR also explicitly defines the flag purpose as detached_merge_request_pipeline and merge_request_pipeline from detached and attached.

What are the relevant issue numbers?

Related gitlab-ee#7380 #58454 (moved)

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