Allow sidekiq workers to set their trace params

It's useful when using `.delay`, cause all jobs are merged into `Sidekiq::Extensions::DelayedClass#perform`.

I use with this code. I can also add it to PR
```ruby
class Sidekiq::Extensions::DelayedClass
  def newrelic_trace_args(msg, queue)
    (target, method_name, args) = YAML.load(msg['args'][0])
    {
      :name => method_name,
      :class_name => target.name,
      :category => 'OtherTransaction/SidekiqJob'
    }
  end
end
```