Move provider plugins into a dedicated directory

Since we will introduce another type of plugin for the policy engine
we want to have each plugin type in separate directories.

We also have to adjust:

- plugin search directories
- po file location
- update paths for calls-doc target

plugins/provider/sip/calls-sip-util.h

@@ -0,0 +1,73 @@
+/*
+ * 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 <sofia-sip/nua.h>
+#include <glib-object.h>
+
+typedef struct {
+  su_home_t  home[1];
+  su_root_t *root;
+} CallsSipContext;
+
+typedef struct {
+  nua_handle_t    *register_handle;
+  nua_handle_t    *call_handle;
+  CallsSipContext *context;
+} CallsSipHandles;
+
+
+/**
+ * SipEngineState:
+ * @SIP_ENGINE_NULL: Not initialized
+ * @SIP_ENGINE_INITIALIZING: Need to complete initialization
+ * @SIP_ENGINE_ERROR: Unrecoverable/Unhandled sofia-sip error
+ * @SIP_ENGINE_READY: Ready for operation
+ */
+typedef enum {
+  SIP_ENGINE_NULL = 0,
+  SIP_ENGINE_INITIALIZING,
+  SIP_ENGINE_ERROR,
+  SIP_ENGINE_READY,
+} SipEngineState;
+
+/**
+ * SipMediaEncryption:
+ * @SIP_MEDIA_ENCRYPTION_NONE: Don't encrypt media streams
+ * @SIP_MEDIA_ENCRYPTION_PREFERRED: Prefer using encryption, but also allow unencrypted media
+ * @SIP_MEDIA_ENCRYPTION_FORCED: Force using encryption, drop unencrypted calls
+ */
+typedef enum {
+  SIP_MEDIA_ENCRYPTION_NONE = 0,
+  SIP_MEDIA_ENCRYPTION_PREFERRED,
+  SIP_MEDIA_ENCRYPTION_FORCED,
+} SipMediaEncryption;
+
+
+gboolean    check_sips                         (const char *addr);
+gboolean    check_ipv6                         (const char *host);
+const char *get_protocol_prefix                (const char *protocol);
+gboolean    protocol_is_valid                  (const char *protocol);