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/Models/CustomSource.cs

@@ -0,0 +1,33 @@
+using System.Text.Json.Serialization;
+
+using AnthropicClient.Utils;
+
+namespace AnthropicClient.Models;
+
+/// <summary>
+/// Represents a custom source that contains a list of text content.
+/// </summary>
+public class CustomSource : Source
+{
+  /// <summary>
+  ///  Gets the list of text content that makes up the custom source.
+  /// </summary>
+  public List<TextContent> Content { get; init; } = [];
+
+  [JsonConstructor]
+  internal CustomSource() : base(SourceType.Content)
+  {
+  }
+
+  /// <summary>
+  /// Initializes a new instance of the <see cref="CustomSource"/> class.
+  /// </summary>
+  /// <returns>A new instance of the <see cref="CustomSource"/> class.</returns>
+  /// <exception cref="ArgumentNullException">Thrown when the content is null.</exception>
+  public CustomSource(List<TextContent> content) : base(SourceType.Content)
+  {
+    ArgumentValidator.ThrowIfNull(content, nameof(content));
+
+    Content = content;
+  }
+}