Real time pipeline show action

app/controllers/projects/pipelines_controller.rb

app/models/ci/group.rb

app/models/ci/stage.rb

app/serializers/job_group_entity.rb

app/serializers/stage_entity.rb

app/serializers/status_entity.rb

changelogs/unreleased/zj-real-time-pipelines.yml

lib/gitlab/ci/status/group/common.rb

lib/gitlab/ci/status/group/factory.rb

lib/gitlab/etag_caching/router.rb

spec/controllers/projects/pipelines_controller_spec.rb

spec/fixtures/api/schemas/pipeline.json

spec/lib/gitlab/ci/status/group/common_spec.rb

spec/lib/gitlab/ci/status/group/factory_spec.rb

spec/models/ci/group_spec.rb

spec/models/ci/stage_spec.rb

spec/serializers/stage_entity_spec.rb