meet the AR interface requirements.

thanks to @chris68 PR #1338

now supports mapped primary key when _id is part of source with an
alias.

Renamed `attributeName` and `className` to `targetAttribute` and `targetClass` for `UniqueValidator` and `ExistValidator`.
Refactored UniqueValidator and ExistValidator.

- allows to have dynamic definition of attributes depended on the
  instance
- there was no real need for it to be static. Places that used it static
  have been refactored.

fixes #1318

fixes #1246

issue #1246

also made everything conistent with client validation

- added example code for application components
- added missing short description

[ci skip]