Add project files.

Lantean.QBitTorrentClient/Converters/CommaSeparatedJsonConverter.cs

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+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Lantean.QBitTorrentClient.Converters
+{
+    internal class CommaSeparatedJsonConverter : JsonConverter<IReadOnlyList<string>>
+    {
+        public override IReadOnlyList<string>? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+        {
+            if (reader.TokenType != JsonTokenType.String)
+            {
+                throw new JsonException("Must be of type string.");
+            }
+
+            List<string> list;
+            var value = reader.GetString();
+            if (value is null)
+            {
+                list = [];
+            }
+            else
+            {
+                var values = value.Split(',');
+                list = [.. values];
+            }
+
+            return list.AsReadOnly();
+        }
+
+        public override void Write(Utf8JsonWriter writer, IReadOnlyList<string> value, JsonSerializerOptions options)
+        {
+            var output = string.Join(',', value);
+
+            writer.WriteStringValue(output);
+        }
+    }
+}