Completion support (#15)

src/mcpdotnet/Protocol/Types/Argument.cs

@@ -0,0 +1,15 @@
+using System.Text.Json.Serialization;
+
+namespace McpDotNet.Protocol.Types;
+
+/// <summary>
+/// Used for completion requests to provide additional context for the completion options.
+/// </summary>
+public class Argument
+{
+    [JsonPropertyName("name")]
+    public string Name { get; set; } = string.Empty;
+
+    [JsonPropertyName("value")]
+    public string Value { get; set; } = string.Empty;
+}