chore: Add custom RuboCop cop to enforce one class per file (#12947)

.rubocop.yml

@@ -7,6 +7,7 @@ plugins:
 require:
   - ./rubocop/use_from_email.rb
   - ./rubocop/custom_cop_location.rb
+  - ./rubocop/one_class_per_file.rb
 
 Layout/LineLength:
   Max: 150
@@ -205,6 +206,9 @@ UseFromEmail:
 CustomCopLocation:
   Enabled: true
 
+Style/OneClassPerFile:
+  Enabled: true
+
 AllCops:
   NewCops: enable
   Exclude: