[#351] Add StringifyPublicKey method for NodeInfo

Signed-off-by: Anton Nikiforov <an.nikiforov@yadro.com>

crypto/util.go

@@ -0,0 +1,9 @@
+package neofscrypto
+
+import "encoding/hex"
+
+// StringifyKeyBinary returns string with HEX representation of source.
+// Format can be changed and it's unsafe to rely on it beyond human-readable output.
+func StringifyKeyBinary(src []byte) string {
+	return hex.EncodeToString(src)
+}