ex_aws_sns: Trusted-attacker `SigningCertURL` permits complete SNS signature bypass

Summary ExAws.SNS.verifymessage/1 fetches the signing certificate from the SigningCertURL field of the incoming SNS message without validating that the URL uses HTTPS or that its host is an AWS-owned SNS certificate domain. An unauthenticated attacker who can POST to any endpoint that calls...