iOS certificate encoding support

[externl]

swift/Ice/info currently fails on iOS

testing connection information... Error: src/Ice/SSL/SecureTransportUtil.cpp:1010 ::Ice::FeatureNotSupportedException IceSSL on iOS does not support encodeCertificate

The encodeCertificate call in Connection.mm throws an exception:

  auto sslInfo = std::dynamic_pointer_cast<Ice::SSL::ConnectionInfo>(infoPtr);
    if (sslInfo)
    {
        std::string encoded;
        if (sslInfo->peerCertificate)
        {
            encoded = Ice::SSL::encodeCertificate(sslInfo->peerCertificate);
        }
        return [factory createSSLConnectionInfo:underlying peerCertificate:toNSString(encoded)];
    }

For now I'm going to ifdef out this section of code for iOS. Could/should we just provide the native certificate instead of this PEM encoded form?