Add CallsCredentials class and use it in CallsSipProvider

CallsCredentials is a dumb container to store account credentials.
This is done in preparation for the account management in the UI.

src/calls-credentials.h

@@ -0,0 +1,42 @@
+/*
+ * Copyright (C) 2021 Purism SPC
+ *
+ * This file is part of Calls.
+ *
+ * Calls is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Calls is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Calls.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Evangelos Ribeiro Tzaras <evangelos.tzaras@puri.sm>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ */
+
+#pragma once
+
+#include <glib-object.h>
+
+G_BEGIN_DECLS
+
+#define CALLS_TYPE_CREDENTIALS (calls_credentials_get_type ())
+
+G_DECLARE_FINAL_TYPE (CallsCredentials, calls_credentials, CALLS, CREDENTIALS, GObject);
+
+
+CallsCredentials       *calls_credentials_new                     ();
+gboolean                calls_credentials_update_from_keyfile     (CallsCredentials *self,
+                                                                   GKeyFile         *key_file,
+                                                                   const char       *name);
+
+G_END_DECLS
+