Abstract base class defines the interface for PKCS#10 key helpers.

This class is fairly dumb, and provides a generic interface for various keystore functionality. The methods are called from the RunL of CPKCS10Request.

The implementation decodes the public key to create a public key object - this is pretty wasteful, as the first thing we're going to do is re-encode it again, mostly in exactly the same format. However it's simpler and less error-prome to do it this way.