Add cop to encourage idempotent Sidekiq workers

This commit's goal is to add a cop (Scalability::IdempotentWorker)
that will catch all workers that don't call idempotent! in its scope.

Calling it will label the worker as idempotent, which will end up in
our Sidekiq logs for further visibility.

This also introduces a shared example and a perform_multiple helper
method for executing jobs multiple times through unit tests.

Also in this commit we set an example in an already idempotent worker
ExpireJobCacheWorker. For this worker we call idempotent! in its
scope and add tests with the new test helpers.

app/workers/admin_email_worker.rb

 # frozen_string_literal: true
-class AdminEmailWorker
+class AdminEmailWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   # rubocop:disable Scalability/CronWorkerContext
   # This worker does not perform work scoped to a context

app/workers/archive_trace_worker.rb

 # frozen_string_literal: true
-class ArchiveTraceWorker
+class ArchiveTraceWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   include PipelineBackgroundQueue

app/workers/authorized_projects_worker.rb

 # frozen_string_literal: true
-class AuthorizedProjectsWorker
+class AuthorizedProjectsWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   prepend WaitableWorker

app/workers/auto_devops/disable_worker.rb

 # frozen_string_literal: true
 module AutoDevops
-  class DisableWorker
+  class DisableWorker # rubocop:disable Scalability/IdempotentWorker
     include ApplicationWorker
     include AutoDevopsQueue

app/workers/auto_merge_process_worker.rb

 # frozen_string_literal: true
-class AutoMergeProcessWorker
+class AutoMergeProcessWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   queue_namespace :auto_merge

app/workers/background_migration_worker.rb

 # frozen_string_literal: true
-class BackgroundMigrationWorker
+class BackgroundMigrationWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   feature_category_not_owned!

app/workers/build_coverage_worker.rb

 # frozen_string_literal: true
-class BuildCoverageWorker
+class BuildCoverageWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   include PipelineQueue

app/workers/build_finished_worker.rb

 # frozen_string_literal: true
-class BuildFinishedWorker
+class BuildFinishedWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   include PipelineQueue

app/workers/build_hooks_worker.rb

 # frozen_string_literal: true
-class BuildHooksWorker
+class BuildHooksWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   include PipelineQueue

app/workers/build_queue_worker.rb

 # frozen_string_literal: true
-class BuildQueueWorker
+class BuildQueueWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   include PipelineQueue

app/workers/build_success_worker.rb

 # frozen_string_literal: true
-class BuildSuccessWorker
+class BuildSuccessWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   include PipelineQueue

app/workers/build_trace_sections_worker.rb

 # frozen_string_literal: true
-class BuildTraceSectionsWorker
+class BuildTraceSectionsWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   include PipelineQueue

app/workers/chaos/cpu_spin_worker.rb

 # frozen_string_literal: true
 module Chaos
-  class CpuSpinWorker
+  class CpuSpinWorker # rubocop:disable Scalability/IdempotentWorker
     include ApplicationWorker
     include ChaosQueue

app/workers/chaos/db_spin_worker.rb

 # frozen_string_literal: true
 module Chaos
-  class DbSpinWorker
+  class DbSpinWorker # rubocop:disable Scalability/IdempotentWorker
     include ApplicationWorker
     include ChaosQueue

app/workers/chaos/kill_worker.rb

 # frozen_string_literal: true
 module Chaos
-  class KillWorker
+  class KillWorker # rubocop:disable Scalability/IdempotentWorker
     include ApplicationWorker
     include ChaosQueue

app/workers/chaos/leak_mem_worker.rb

 # frozen_string_literal: true
 module Chaos
-  class LeakMemWorker
+  class LeakMemWorker # rubocop:disable Scalability/IdempotentWorker
     include ApplicationWorker
     include ChaosQueue

app/workers/chaos/sleep_worker.rb

 # frozen_string_literal: true
 module Chaos
-  class SleepWorker
+  class SleepWorker # rubocop:disable Scalability/IdempotentWorker
     include ApplicationWorker
     include ChaosQueue

app/workers/chat_notification_worker.rb

 # frozen_string_literal: true
-class ChatNotificationWorker
+class ChatNotificationWorker # rubocop:disable Scalability/IdempotentWorker
   include ApplicationWorker
   TimeoutExceeded = Class.new(StandardError)

app/workers/ci/archive_traces_cron_worker.rb

 # frozen_string_literal: true
 module Ci
-  class ArchiveTracesCronWorker
+  class ArchiveTracesCronWorker # rubocop:disable Scalability/IdempotentWorker
     include ApplicationWorker
     include CronjobQueue # rubocop:disable Scalability/CronWorkerContext