model: Add ChannelPostPolicy.fromApiValue method

Author: sm-sayedi

lib/api/model/model.dart

@@ -382,6 +382,11 @@ enum ChannelPostPolicy {
   final int? apiValue;
 
   int? toJson() => apiValue;
+
+  static ChannelPostPolicy fromApiValue(int value) => _byApiValue[value]!;
+
+  static final _byApiValue = _$ChannelPostPolicyEnumMap
+    .map((key, value) => MapEntry(value, key));
 }
 
 /// As in `subscriptions` in the initial snapshot.