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

tests/AnthropicClient.Tests/Unit/Models/MessageRequestTests.cs

@@ -558,8 +558,9 @@ public class MessageRequestTests : SerializationTest
 
     var imageContent = messageRequest.Messages[0].Content[0] as ImageContent;
     imageContent!.Type.Should().Be("image");
-    imageContent.Source.MediaType.Should().Be("image/jpeg");
-    imageContent.Source.Data.Should().Be("data");
+
+    imageContent.Source.As<ImageSource>().MediaType.Should().Be("image/jpeg");
+    imageContent.Source.As<ImageSource>().Data.Should().Be("data");
   }
 
   [Fact]