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/Base64Source.cs

@@ -0,0 +1,39 @@
+using System.Text.Json.Serialization;
+
+using AnthropicClient.Utils;
+
+namespace AnthropicClient.Models;
+
+/// <summary>
+/// Represents an base64 source.
+/// </summary>
+public class Base64Source : Source
+{
+  /// <summary>
+  /// Gets the media type of the source.
+  /// </summary>
+  [JsonPropertyName("media_type")]
+  public string MediaType { get; init; } = string.Empty;
+
+  /// <summary>
+  /// Gets the data of the source.
+  /// </summary>
+  public string Data { get; init; } = string.Empty;
+
+  /// <summary>
+  /// Initializes a new instance of the <see cref="Base64Source"/> class.
+  /// </summary>
+  /// <param name="mediaType">The media type of the source.</param>
+  /// <param name="data">The data of the source.</param>
+  /// <exception cref="ArgumentException">Thrown when the media type is invalid.</exception>
+  /// <exception cref="ArgumentNullException">Thrown when the media type or data is null.</exception>
+  /// <returns>A new instance of the <see cref="Base64Source"/> class.</returns>
+  public Base64Source(string mediaType, string data) : base(SourceType.Base64)
+  {
+    ArgumentValidator.ThrowIfNull(mediaType, nameof(mediaType));
+    ArgumentValidator.ThrowIfNull(data, nameof(data));
+
+    MediaType = mediaType;
+    Data = data;
+  }
+}