Support custom attributes types

[akostadinov]

Steps to reproduce

1. have a CLOB column type
2. have a field with custom attribute, e.g. class MyCustomAttribute < ActiveRecord::Type::Text
3. try to use attribute

my_model.myattribute = "whatever long serialized string ..."
my_model.save!
my_model.myattribute
=> whatever deserialized object as expected
my_model.reload
my_model.myattribute

Expected behavior

=> whatever deserialized object as expected

Basically everything works as if ActiveRecord::Type::Text has been used.

Actual behavior

=> "#<MyDeserializedClass:0x0000000010967bd8>tring ..."

System configuration

Rails version: 5.0.7.2

Oracle enhanced adapter version: activerecord-oracle_enhanced-adapter (1.7.11)

Ruby version: ruby 2.5.9p229bu

Oracle Database version: oracle:19.3.0