Improve progress reporting (#145)

* Improve progress reporting

- Enable injecting an `IProgress<>` into a tool call to report progress when the client has supplied a progress token
- Add a custom type for ProgressToken and ProgressNotification to map to the schema
- Simplify RequestId to match ProgressToken's shape
- Serialize StdioClientTransport's SendMessageAsync implementation
- Add strongly-typed names for all request methods
- Change all request IDs created by McpJsonRpcEndpoint to include a guid for that endpoint

* Address PR feedback

src/ModelContextProtocol/ProgressNotificationValue.cs

@@ -0,0 +1,14 @@
+namespace ModelContextProtocol;
+
+/// <summary>Provides a progress value that can be sent using <see cref="IProgress{ProgressNotificationValue}"/>.</summary>
+public record struct ProgressNotificationValue
+{
+    /// <summary>Gets or sets the progress thus far.</summary>
+    public required float Progress { get; init; }
+
+    /// <summary>Gets or sets the total number of items to process (or total progress required), if known.</summary>
+    public float? Total { get; init; }
+
+    /// <summary>Gets or sets an optional message describing the current progress.</summary>
+    public string? Message { get; init; }
+}