Set X-GitLab-NotificationReason header if notification reason is explicit subscription

What does this MR do?

What the title says. :) This can be used in email filters to differentiate notification emails of stuff you have explicitly subscribed to.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #34416 (closed)