Add debug prints to Sign()

Print out original plaintext and digest when in
debug mode.
Helps SDK developers when debugging signature
issues as they can compare values on either side
of gRPC

Change-Id: I6286f0bf20a6ab71e72e8ddedcf167c661b41dcb
Signed-off-by: tuand27613 <tdang@us.ibm.com>

msp/identities.go

@@ -157,6 +157,9 @@ func (id *signingidentity) Sign(msg []byte) ([]byte, error) {
 		return nil, fmt.Errorf("Failed computing digest [%s]", err)
 	}
 
+	mspLogger.Debugf("Sign: plaintext: %X \n", msg)
+	mspLogger.Debugf("Sign: digest: %X \n", digest)
+
 	// Sign
 	return id.signer.Sign(rand.Reader, digest, nil)
 }