Create a notification subscription

Subscribe to receiving notifications at a given endpoint. The endpoint should be able to handle AWS SNS subscription requests. For more details see https://docs.aws.amazon.com/mobile/sdkforxamarin/developerguide/sns-send-http.html. Note, the sandbox environment allows a maximum of 3 active subscriptions; otherwise, this is limited to 1 active subscription and subsequent create requests will be rejected with a Limit Exceeded error.