feat: add support for citations

BREAKING CHANGE: Changed type of public Source properties.
Updated `Source` property on `DocumentContent` and `ImageContent`
types to use base `Source` type

- include support for citing custom sources
- include support for citing PDF sources
- include support for citing text sources

src/AnthropicClient/Json/JsonSerializationOptions.cs

@@ -19,6 +19,7 @@ static class JsonSerializationOptions
       new JsonStringEnumConverter(),
       new MessageBatchResultConverter(),
       new CitationConverter(),
+      new SourceConverter(),
     },
     DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
   };

src/AnthropicClient/Json/SourceConverter.cs

@@ -0,0 +1,46 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+using AnthropicClient.Models;
+
+namespace AnthropicClient.Json;
+
+class SourceConverter : JsonConverter<Source>
+{
+  public override Source Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+  {
+    using var jsonDocument = JsonDocument.ParseValue(ref reader);
+    var root = jsonDocument.RootElement;
+    var type = root.GetProperty("type").GetString();
+    return type switch
+    {
+      SourceType.Text => JsonSerializer.Deserialize<TextSource>(root.GetRawText(), options)!,
+      SourceType.Content => JsonSerializer.Deserialize<CustomSource>(root.GetRawText(), options)!,
+      SourceType.Base64 => JsonSerializer.Deserialize<Base64Source>(root.GetRawText(), options)!,
+      _ => throw new JsonException($"Unknown content type: {type}")
+    };
+  }
+
+  public override void Write(Utf8JsonWriter writer, Source value, JsonSerializerOptions options)
+  {
+    if (value is TextSource textSource)
+    {
+      JsonSerializer.Serialize(writer, textSource, options);
+      return;
+    }
+
+    if (value is CustomSource customSource)
+    {
+      JsonSerializer.Serialize(writer, customSource, options);
+      return;
+    }
+
+    if (value is Base64Source base64Source)
+    {
+      JsonSerializer.Serialize(writer, base64Source, options);
+      return;
+    }
+
+    JsonSerializer.Serialize(writer, value, value.GetType(), options);
+  }
+}