Add custom data helper

[tombruijn]

Our sample data docs give this example.

Appsignal::Transaction.current.set_sample_data(
  "custom_data",
  :i18n => {
    :locale => "en_GB",
    :default_locale => "en_US"
  }
)

Let's add a custom helper for this like Appsignal.set_custom_data and Appsignal::Transaction#set_custom_data