void verifyCertUrl(URI certUrl) {
        assertIsHttps(certUrl);
        assertIsFromSns(certUrl);
    }
Verifies the location of the signing cert is secure and from SNS.

@param certUrl URL to verify.
@throws SdkClientException If signing URL cannot be trusted.
aws-java-sdk-sns/src/main/java/com/amazonaws/services/sns/message/SigningCertUrlVerifier.java