Add exactly_one_of validator for ActiveRecord models

Follow-up from !198202 (comment 2659472924)

We do use a validation like this in multiple places, so it would be great if we can have a validator for this and reuse everywhere.

Implementation plan

Something like this could work, but definitively open to any changes in the snippet below

class OnlyOnePresentValidator < ActiveModel::Validator
  def validate(record)
    fields = options[:fields]
    values = fields.map { |field| record.public_send(field) }.compact

    unless values.one?
      record.errors.add(:base, "Exactly one of #{fields.join(', ')} must be present")
    end
  end
end

validates_with OnlyOnePresentValidator, fields: [:project_id, :group_id, :organization_id]

Support contact: @mcelicalderonG