Add pipeline.default_branch? mehod

What does this MR do?

Helps to know if the pipeline ref matches the project's default branch 8661a35e

This is a backport of changes made in this EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7578

What are the relevant issue numbers?

gitlab-org/gitlab-ee#6717

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides