tech(expires-user.cron): ajoute une tache cron [débrayable] pour faire tourner la tache de suppression des usagers inutile

attention, pour notre instance, avec ma db anonymiser :
```
ExpiredUsersDeletionService.find_expired_user.pluck(:id).size
=> 1795515
```

app/jobs/cron/expired_users_deletion_job.rb

@@ -0,0 +1,8 @@
+class Cron::ExpiredUsersDeletionJob < Cron::CronJob
+  self.schedule_expression = "every day at 11 pm"
+
+  def perform(*args)
+    return if ENV['EXPIRE_USER_DELETION_JOB_DISABLED'].present?
+    ExpiredUsersDeletionService.process_expired
+  end
+end

config/env.example.optional

@@ -253,3 +253,6 @@ BULK_EMAIL_QUEUE="low_priority"
 
 # work in progress about attestation_v2
 WEASYPRINT_URL="http://10.33.23.204:5000/pdf"
+
+# Use this env var to disable the user expiration mechanism
+EXPIRE_USER_DELETION_JOB_DISABLED=anything

spec/jobs/cron/expired_users_deletion_job_spec.rb

@@ -0,0 +1,19 @@
+describe Cron::ExpiredUsersDeletionJob do
+  subject { described_class.perform_now }
+
+  context 'when env[EXPIRE_USER_DELETION_JOB_DISABLED] is present' do
+    before { expect(ENV).to receive(:[]).with('EXPIRE_USER_DELETION_JOB_DISABLED').and_return('anything') }
+
+    it 'does not call ExpiredUsersDeletionService.process_expired' do
+      expect(ExpiredUsersDeletionService).not_to receive(:process_expired)
+      subject
+    end
+  end
+
+  context 'when env[EXPIRE_USER_DELETION_JOB_DISABLED] is absent' do
+    it 'calls ExpiredUsersDeletionService.process_expired' do
+      expect(ExpiredUsersDeletionService).to receive(:process_expired)
+      subject
+    end
+  end
+end