Yield publisher hash to block as first positional argument

#### Motivation (Why)

Allows calling `Publisher.stop(id: publisher[:id])` to help with TDD but also offer a more controlled way outside of infrastructure layer / signals e.g.

```ruby
Outboxer::Publisher.publish_messages do |publisher, messages|
  # TODO: publish batch of messages to broker

  Publisher.stop(id: publisher[:id])
end
```
