Add sdp transformers (#350)

* Added custom options to call()

* Added offer modification ability to _createLocalDescription()

---------

Co-authored-by: Perondas <Pperondas@gmail.com>

Author: Perondas

lib/src/map_helper.dart

@@ -0,0 +1,20 @@
+class MapHelper {
+  /// Merge two maps recursively. If the same key exists in both maps, the value
+  /// from the second map will be used. If the value is a map, the merge will be
+  /// recursive. Sub-maps also have to have their keys be of type [K].
+  static Map<K, dynamic> merge<K>(Map<K, dynamic> a, Map<K, dynamic> b) {
+    for (K key in b.keys) {
+      if (a.containsKey(key)) {
+        if (a[key] is Map && b[key] is Map) {
+          a[key] = merge(a[key] as Map<K, dynamic>, b[key] as Map<K, dynamic>);
+        } else {
+          a[key] = b[key]!;
+        }
+      } else {
+        a[key] = b[key]!;
+      }
+    }
+
+    return a;
+  }
+}

lib/src/rtc_session.dart

@@ -1608,6 +1608,12 @@ class RTCSession extends EventManager implements Owner {
     Completer<RTCSessionDescription> completer =
         Completer<RTCSessionDescription>();
 
+    List<Future<RTCSessionDescription> Function(RTCSessionDescription)>
+        _modifiers = constraints?['offerModifiers'] ??
+            <Future<RTCSessionDescription> Function(RTCSessionDescription)>[];
+
+    constraints?['offerModifier'] = null;
+
     if (type != 'offer' && type != 'answer') {
       completer.completeError(Exceptions.TypeError(
           'createLocalDescription() | invalid type "$type"'));
@@ -1638,6 +1644,11 @@ class RTCSession extends EventManager implements Owner {
     // Add 'pc.onicencandidate' event handler to resolve on last candidate.
     bool finished = false;
 
+    for (Future<RTCSessionDescription> Function(RTCSessionDescription) modifier
+        in _modifiers) {
+      desc = await modifier(desc);
+    }
+
     Future<void> ready() async {
       if (!finished && _status != C.STATUS_TERMINATED) {
         finished = true;

lib/src/sip_ua_helper.dart

@@ -2,6 +2,7 @@ import 'dart:async';
 
 import 'package:flutter_webrtc/flutter_webrtc.dart';
 import 'package:logger/logger.dart';
+import 'package:sip_ua/src/map_helper.dart';
 
 import 'config.dart';
 import 'constants.dart' as DartSIP_C;
@@ -84,9 +85,13 @@ class SIPUAHelper extends EventManager {
   Future<bool> call(String target,
       {bool voiceonly = false,
       MediaStream? mediaStream,
-      List<String>? headers}) async {
+      List<String>? headers,
+      Map<String, dynamic>? customOptions}) async {
     if (_ua != null && _ua!.isConnected()) {
       Map<String, dynamic> options = buildCallOptions(voiceonly);
+      if (customOptions != null) {
+        options = MapHelper.merge(options, customOptions);
+      }
       if (mediaStream != null) {
         options['mediaStream'] = mediaStream;
       }