Add request options bag to high level requests and include Meta (#970)

Co-authored-by: Stephen Toub <stoub@microsoft.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/ModelContextProtocol.Core/RequestOptions.cs

@@ -0,0 +1,96 @@
+using System.Text.Json;
+using System.Text.Json.Nodes;
+using ModelContextProtocol.Protocol;
+
+namespace ModelContextProtocol;
+
+/// <summary>
+/// Contains optional parameters for MCP requests.
+/// </summary>
+public sealed class RequestOptions
+{
+    /// <summary>
+    /// Optional metadata to include in the request.
+    /// </summary>
+    private JsonObject? _meta;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="RequestOptions"/> class.
+    /// </summary>
+    public RequestOptions()
+    {
+    }
+
+    /// <summary>
+    /// Optional metadata to include in the request.
+    /// When getting, automatically includes the progress token if set.
+    /// </summary>
+    public JsonObject? Meta
+    {
+        get => _meta ??= [];
+        set
+        {
+            // Capture the existing progressToken value if set.
+            var existingProgressToken = _meta?["progressToken"];
+
+            if (value is not null)
+            {
+                if (existingProgressToken is not null)
+                {
+                    value["progressToken"] ??= existingProgressToken;
+                }
+
+                _meta = value;
+            }
+            else if (existingProgressToken is not null)
+            {
+                _meta = new()
+                {
+                    ["progressToken"] = existingProgressToken,
+                };
+            }
+            else
+            {
+                _meta = null;
+            }
+        }
+    }
+
+    /// <summary>
+    /// The serializer options governing tool parameter serialization. If null, the default options are used.
+    /// </summary>
+    public JsonSerializerOptions? JsonSerializerOptions { get; set; }
+
+    /// <summary>
+    /// The progress token for tracking long-running operations.
+    /// </summary>
+    public ProgressToken? ProgressToken
+    {
+        get
+        {
+            return _meta?["progressToken"] switch
+            {
+                JsonValue v when v.TryGetValue(out string? s) => new(s),
+                JsonValue v when v.TryGetValue(out long l) => new(l),
+                _ => null
+            };
+        }
+        set
+        {
+            if (value?.Token is { } token)
+            {
+                _meta ??= [];
+                _meta["progressToken"] = token switch
+                {
+                    string s => s,
+                    long l => l,
+                    _ => throw new InvalidOperationException("ProgressToken must be a string or long"),
+                };
+            }
+            else
+            {
+                _meta?.Remove("progressToken");
+            }
+        }
+    }
+}