Add back decorate_attribute_type

Originally #39882 is intended to remove `decorate_matching_attribute_type`
but leave `decorate_attribute_type` at that point, but in the
disscussion in #39882, to demonstrate removing `AttributeDecorators`
module, I've removed all decoration methods.

I wrote user-defined type decoration as inline, I wasn't sure that is
valid (meaningful) usage (that is a reason why I wasn't in a hurry to
remove that method), as I said at https://github.com/rails/rails/pull/39882#discussion_r457750898.

In #39897 and #39902, `enum` also need to support user-defined type
decoration, so I've decided to add back `decorate_attribute_type` as
before #39882.

activerecord/lib/active_record/attribute_methods/serialization.rb

@@ -69,14 +69,7 @@ module ActiveRecord
             Coders::YAMLColumn.new(attr_name, class_name_or_coder)
           end
 
-          attr_name = attr_name.to_s
-          type, options = attributes_to_define_after_schema_loads[attr_name]
-
-          attribute(attr_name) do |cast_type|
-            if type && !type.is_a?(Proc)
-              cast_type = _lookup_cast_type(attr_name, type, options)
-            end
-
+          decorate_attribute_type(attr_name.to_s) do |cast_type|
             if type_incompatible_with_serialize?(cast_type, class_name_or_coder)
               raise ColumnNotSerializableError.new(attr_name, cast_type)
             end

activerecord/lib/active_record/attributes.rb

@@ -270,6 +270,18 @@ module ActiveRecord
           _default_attributes[name] = default_attribute
         end
 
+        def decorate_attribute_type(attr_name, **default)
+          type, options = attributes_to_define_after_schema_loads[attr_name]
+
+          attribute(attr_name, **default) do |cast_type|
+            if type && !type.is_a?(Proc)
+              cast_type = _lookup_cast_type(attr_name, type, options)
+            end
+
+            yield cast_type
+          end
+        end
+
         def _lookup_cast_type(name, type, options)
           case type
           when Symbol

activerecord/lib/active_record/enum.rb

@@ -183,13 +183,8 @@ module ActiveRecord
         detect_enum_conflict!(name, "#{name}=")
 
         attr = attribute_alias?(name) ? attribute_alias(name) : name
-        type, options = attributes_to_define_after_schema_loads[attr]
-
-        attribute(attr, **default) do |subtype|
-          if type && !type.is_a?(Proc)
-            subtype = _lookup_cast_type(attr, type, options)
-          end
 
+        decorate_attribute_type(attr, **default) do |subtype|
           EnumType.new(attr, enum_values, subtype)
         end