should have validator helper for extensions to validate their associated settings

[hoatle]

Currently, the settings are not strictly validated and this could cause confusions to users when something wrong happens.

Learn from:

• http://juixe.com/techknow/index.php/2006/07/29/rails-model-validators/
• https://guides.rubyonrails.org/active_record_validations.html
• https://api.rubyonrails.org/classes/ActiveModel/Validator.html
• https://pdabrowski.com/blog/ruby-on-rails/validations/building-custom-validator/

The spec should be done first before implementing.

[hoatle]

we can use https://github.com/ruby-json-schema/json-schema

[hoatle]

we should work on this extension and use it https://github.com/teracyhq-incubator/teracy-dev-validator If the extension can provide all the functionality, we can use that extension instead of add more code to the kernel