@param [String] topic_arn An AWS SNS Topic ARN. It should be
formatted something like: arn:aws:sns:us-east-1:123456789012:TopicName
@return [Topic] Returns a topic with the given Topic ARN.
Creates and returns a new SNS Topic. @return [Topic] Returns a new topic with the given name.
Yields once for each topic. @yieldparam [Topic] topic @return [nil]
[Validate]