Move all CI files under `Ci::` namespace

Spelunking the code base I've found a lot of classes that are related to CI domain which are not nested inside Ci:: namespace. I think we should start migrating all these classes under the namespace and start enforcing it in reviews.

Having all the CI related classes under the namespace

• makes it clear who is the responsible team
• when touching a non namespaced class will be clear to us that we are reaching outside the domain, hence take the necessary considerations
• lowers the barrier for a new team member or community contributor to understand the boundaries

Here are the classes I found so far (probably missing some). We could move them slowly bit by bit in several cohesive MRs. Moving workers is a lot more delicate, and we have many of them.

• concern HasRef is used only for CI models !27056 (merged)
• concern HasStatus !34577 (merged)
• concerns HasVariable and NewHasVariable !27306 (merged)
• concern Maskable !27305 (merged)
• CommitWithPipeline could remain outside CI but CommitWithPipeline#pipelines should not know about Ci::Pipeline details.
• JobsFinder !27190 (merged)
• PipelineSchedulesFinder !27190 (merged)
• PipelinesFinder !27190 (merged)
• RunnerJobsFinder !27190 (merged)
• ArtifactsFinder !28233 (merged)
• CommitStatusEnums
• EE::ProcessGithubPullRequestEventService !28517 (merged)
• ExternalPullRequest
• ExternalPullRequests::CreatePipelineService !28517 (merged)
• BuildsHelper !35079 (merged)
• CiStatusHelper !35073 (merged)
• CiVariablesHelper!35199 (merged)
• JobsHelper !35389 (merged)
• PipelineSchedulesHelper !36130 (merged)
• RunnersHelper !36144 (merged)
• TriggersHelper
• ArchiveTraceWorker
• BuildCoverageWorker
• BuildFinishedWorker
• BuildHooksWorker
• BuildQueueWorker
• BuildsSuccessWorker
• BuildTraceSectionsWorker
• CreatePipelineWorker
• ExpireBuildArtifactsWorker
• ExpireBuildInstanceArtifactsWorker
• ExpireJobCacheWorker
• ExpirePipelineCacheWorker
• ExpirePipelineHooksWorker
• PipelineMetricsWorker
• PipelineNotificationWorker
• PipelineProcessWorker
• PipelineScheduleWorker
• PipelineSuccessWorker
• PipelineUpdateWorker
• RunPipelineScheduleWorker
• StageUpdateWorker
• StuckCiJobsWorker
• UpdateExternalPullRequestsWorker
• EE::JobsHelper !35389 (merged)
• EE::RunnersHelper !36144 (merged)
• Entities::JobRequest::* from lib/api/entities/job_request -> lib/api/entities/ci/job_request
• Entities::JobArtifactFile, Entities::JobArtifact !37981 (diffs)
• Entities::JobBasicWithProject, Entities::JobBasic, Entities::Job !37981 (diffs)
• Entities::PipelineBasic, Entities::PipelineScheduleDetails, Entities::PipelineSchedule, Entities::Pipeline !37981 (diffs)
• Entities::Runner, Entities::RunnerDetails, Entities::RunnerRegistrationDetails
• Entities::Variable !37981 (diffs)
• API::Helpers::Runner
• API::JobArtifacts
• API::Jobs
• API::PipelineSchedules !36381 (merged)
• API::Pipelines !36381 (merged)
• API::Runner !35191 (merged)
• API::Runners !35191 (merged)
• API::Triggers
• API::Variables
• Gitlab::Artifacts::MigrationHelper
• Gitlab::Badge::*
• EE::API::Helpers::Runner
• EE::API::JobArtifacts
• VariableSerializer !36636 (merged)
• VariableEntity !36636 (merged)
• GroupVariableSerializer !36636 (merged)
• GroupVariableEntity !36636 (merged)

Move related specs by consequence.

Can't be easily moved - needs more thoughts

• CommitStatus + GenericCommitStatus because we would need to migrate the type column in ci_builds. We could move it under Ci:: and leave CommitStatus as inheritance only class from Ci::CommitStatus that would allow us to read old records until the next major release.
• Entities::CommitStatus
• API::CommitStatuses