Expose additional merge request pipeline variables

What does this MR do?

New predefined variables for MR pipelines

Variable name	Description	Sample value
CI_MERGE_REQUEST_TITLE	The Title of the merge request	WIP: Expose additional merge request pipeline variables
CI_MERGE_REQUEST_ASSIGNEES	The Assignee of the merge request	root
CI_MERGE_REQUEST_MILESTONE	The Milestone of the merge request	v1.0
CI_MERGE_REQUEST_LABELS	The Labels of the merge request	bug,feature

What are the relevant issue numbers?

#55777 (moved), #14174 (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