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/Citation.cs

@@ -0,0 +1,32 @@
+using System.Text.Json.Serialization;
+
+namespace AnthropicClient.Models;
+
+/// <summary>
+/// Represents a citation
+/// </summary>
+public abstract class Citation
+{
+  /// <summary>
+  /// Gets the type of the citation.
+  /// </summary>
+  public string Type { get; init; } = string.Empty;
+
+  /// <summary>
+  /// Gets the text that is cited.
+  /// </summary>
+  [JsonPropertyName("cited_text")]
+  public string CitedText { get; init; } = string.Empty;
+
+  /// <summary>
+  /// Gets the document index of the citation.
+  /// </summary>
+  [JsonPropertyName("document_index")]
+  public int DocumentIndex { get; init; }
+
+  /// <summary>
+  /// Gets the title of the document from which the citation is made.
+  /// </summary>
+  [JsonPropertyName("document_title")]
+  public string DocumentTitle { get; init; } = string.Empty;
+}