Simplify the Validator.validate() return type

[vlukashov]

The return type of the validate() method in record-level validators should be more intuitive to use (should not require including boilerplate / unnecessary properties).

DoD:

• it is possible to mark a single model property as 'invalid': return { property: model.password }
• it is possible to mark several model properties as 'invalid': return [{ property: model.password }, { property: model.repeatPassword }] (in this case the validator's default message it linked to the first property in the array)
• it is possible to provide different messages for each invalid model property: [{ property: model.password }, { property: model.repeatPassword, message: 'please check that you've repeated the password exactly as it's typed in the field above ^' }]
• it is possible to mark the entire record as invalid without marking any specific field as invalid: return false
• the validate method has to always return something, an empty return is not OK: return true or return [] both are treated as 'validation passed without errors' (the recommended way in all our examples is return true).

Extra:
It’s not easy to access the model inside the validator function => add binder as the second parameter to the validator callback: validate: async (value, binder) => { ... }