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/TextSource.cs

@@ -0,0 +1,33 @@
+using System.Text.Json.Serialization;
+
+namespace AnthropicClient.Models;
+
+/// <summary>
+/// Represents a text document source.
+/// </summary>
+public class TextSource : Source
+{
+  /// <summary>
+  /// Gets the media type of the source.
+  /// </summary>
+  [JsonPropertyName("media_type")]
+  public string MediaType { get; } = "text/plain";
+
+  /// <summary>
+  /// Gets the data of the source.
+  /// </summary>
+  public string Data { get; init; } = string.Empty;
+
+  /// <summary>
+  /// Initializes a new instance of the <see cref="TextSource"/> class.
+  /// </summary>
+  /// <param name="data">The data of the document.</param>
+  /// <exception cref="ArgumentNullException">Thrown when the data is null.</exception>
+  /// <returns>A new instance of the <see cref="TextSource"/> class.</returns>
+  public TextSource(string data) : base(SourceType.Text)
+  {
+    Data = data;
+  }
+}
+
+