Introduce typed event payload

Hello!

I found interesting example in presentation of Nikita Shilnikov

```
module Events
class PasswordChanged < Dry::Struct
attribute :user_id, Types::UUID
attribute :password, Types::String
end

class EmailChanged < Dry::Struct
attribute :user_id, Types::UUID
attribute :email, Types::String
end

Updated = PasswordChanged | EmailChanged
end
```

What do you think to use Dry::Struct for events?

IMHO it will make an application (where you will use events) more clear, we will see all possible events, and that events will have guaranteed shape.