DNSSEC: add knot_dnssec_sign_key() function

refs #2413

Change-Id: I8c40b8bdf39bff7cf4e5d6ae41cb2612b634b932

src/libknot/sign/dnssec.c

@@ -732,6 +732,17 @@ size_t knot_dnssec_sign_size(knot_dnssec_key_t *key)
 	return key->data->functions->sign_size(key);
 }
 
+/*!
+ * \brief Get DNSSEC key used by given signing context.
+ */
+const knot_dnssec_key_t *knot_dnssec_sign_key(knot_dnssec_sign_context_t *context)
+{
+	if (!context)
+		return NULL;
+
+	return context->key;
+}
+
 /*!
  * \brief Add data into DNSSEC signature.
  */