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/Source.cs

@@ -0,0 +1,22 @@
+namespace AnthropicClient.Models;
+
+/// <summary>
+/// Represents a base class for sources.
+/// </summary>
+public abstract class Source
+{
+  /// <summary>
+  /// Gets the type of the source.
+  /// </summary>
+  public string Type { get; init; }
+
+  /// <summary>
+  /// Initializes a new instance of the <see cref="Source"/> class.
+  /// </summary>
+  /// <param name="type">The type of the source.</param>
+  /// <returns>A new instance of the <see cref="Source"/> class.</returns>
+  protected Source(string type)
+  {
+    Type = type;
+  }
+}