Verifies an endpoint owner's intent to receive messages by validating the token sent to the endpoint by an earlier Subscribe
action. If the token is valid, the action creates a new subscription and returns its Amazon Resource Name (ARN). This call requires an AWS signature only when the AuthenticateOnUnsubscribe
flag is set to "true".
/// Indicates that the user has been denied access to the requested resource.
///
/// Indicates an internal service error.
///
/// Indicates that a request parameter does not comply with the associated constraints.
///
/// Indicates that the requested resource does not exist.
///
/// Indicates that the customer already owns the maximum allowed number of subscriptions.
///