message: use AccountId instead of String for account
diff: @@ -404,7 +404,7 @@ pub struct WebhookEvent {
  /// The connected account that originated the event.
  #[serde(skip_serializing_if = "Option::is_none")]
  - pub account: Option<String>,
  + pub account: Option<AccountId>,
  /// The Stripe API version used to render `data`.
  ///
  