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/CharacterLocationCitation.cs

@@ -0,0 +1,21 @@
+using System.Text.Json.Serialization;
+
+namespace AnthropicClient.Models;
+
+/// <summary>
+/// Represents a citation for specific locations within text content.
+/// </summary>
+public class CharacterLocationCitation : Citation
+{
+  /// <summary>
+  /// Gets the start character index of the citation.
+  /// </summary>
+  [JsonPropertyName("start_char_index")]
+  public int StartCharIndex { get; init; }
+
+  /// <summary>
+  /// Gets the end character index of the citation.
+  /// </summary>
+  [JsonPropertyName("end_char_index")]
+  public int EndCharIndex { get; init; }
+}