Add structured output/output schema support for server-side tools. (#480)

* Add structured output/output schema support for server-side tools.

* Address feedback.

* Add McpServerToolAttribute.UseStructuredContent

---------

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol.Core/Protocol/CallToolResponse.cs

@@ -1,3 +1,4 @@
+using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
 
 namespace ModelContextProtocol.Protocol;
@@ -27,6 +28,12 @@ public class CallToolResponse
     [JsonPropertyName("content")]
     public List<Content> Content { get; set; } = [];
 
+    /// <summary>
+    /// Gets or sets an optional JSON object representing the structured result of the tool call.
+    /// </summary>
+    [JsonPropertyName("structuredContent")]
+    public JsonNode? StructuredContent { get; set; }
+
     /// <summary>
     /// Gets or sets an indication of whether the tool call was unsuccessful.
     /// </summary>