message: Add product field to Plan object.
  According to a plan object always
  contains a field `product` containing the id of the product the plan is
  a part of.
  This commit adds the `product` field to the Plan struct, with the goal
  of enabling simpler handling of subscription flows when using webhooks.
diff: @@ -43,6 +43,7 @@ pub struct Plan {
  pub livemode: bool,
  pub metadata: Metadata,
  pub nickname: Option<String>,
  + pub product: String,
  pub statement_descriptor: Option<String>,
  pub trial_period_days: Option<u64>,
  }
  