Add first custom validator for new ci config

This follows a standard `ActiveModel` pattern of creating a custom
validators. We use `ActiveModel::EachValidator` here that reuses methods
provided by `LegacyValidationHelpers`.

We will remove `LegacyValidationHelpers` on some point in the future, at
the later stages of CI configuration refactoring. It may be possible
to rewrite custom validators to use format like:

`validates :config, array_of: String`