cpp: Add getters for the attributes of a trust signature
* lang/cpp/src/key.h (TrustSignatureTrust): New enum. * lang/cpp/src/key.h, lang/cpp/src/key.cpp (UserID::Signature::isTrustSignature): New. (UserID::Signature::trustValue): New. (UserID::Signature::trustDepth): New. (UserID::Signature::trustScope): New.