Pipeline status subscription

Summary

The purpose of this issue is to add a new graphql subscription for pipeline status details. This initial implementation will only be concerned with pipeline status details available on the latest commit header in the main repository view

Proposal

• feature flag real_time_ci_status
• add pipeline_status_updated field to subscription type
• add PipelineStatusUpdated subscription class
• add feature flagged trigger definition/method
• add trigger execution in most appropriate existing code (ie in a service where pipeline status/details are updated)

Implementation Table

Group	Issue Link
backend	👈 You are here
frontend	#516239 (closed)

Documentation

GraphQL API Style guide#Subscriptions