Checks if the provided atSign's atServer has been properly activated with a public RSA key.
atClient must be authenticated
atSign is the atSign to check
Returns true, if the atSign's cloud secondary server has an existing public:publickey@ in their server,
Returns false, if the atSign's cloud secondary exists, but does not have an existing public:publickey@
Throws AtClientException if the cloud secondary is invalid or not reachable