core: pass hints as strv to nm_act_request_get_secrets()

Extend nm_act_request_get_secrets() API to allow for the underlying
flexibility (of the API that it calls) to accept a strv list of hints.

src/nm-act-request.h

@@ -69,7 +69,7 @@ NMActRequestGetSecretsCallId *nm_act_request_get_secrets (NMActRequest *req,
                                                           gboolean take_ref,
                                                           const char *setting_name,
                                                           NMSecretAgentGetSecretsFlags flags,
-                                                          const char *hint,
+                                                          const char *const*hints,
                                                           NMActRequestSecretsFunc callback,
                                                           gpointer callback_data);