Adding a base class for request params that exposes the progressToken (#99)

* Adding a base class for request params that exposes the progressToken

This aligns more with the type structure see in the other SDK's (eg: https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/types.py#L41) while also ensuring they are exposing the JSONRPCRequest (https://github.com/modelcontextprotocol/specification/blob/main/schema/2024-11-05/schema.json#L771-L806) data.

Storing progressToken as an object as it could be an int or string (per https://github.com/modelcontextprotocol/specification/blob/main/schema/2024-11-05/schema.json#L1268-L1274).

Fixes #67

* Using usings

* Apply suggestions from code review

---------

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

src/ModelContextProtocol/Protocol/Types/RequestParams.cs

@@ -0,0 +1,16 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol.Types;
+
+/// <summary>
+/// Base class for all request parameters.
+/// <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/2024-11-05/schema.json#L771-L806">See the schema for details</see>
+/// </summary>
+public abstract class RequestParams
+{
+    /// <summary>
+    /// Metadata related to the tool invocation.
+    /// </summary>
+    [JsonPropertyName("_meta")]
+    public RequestParamsMetadata? Meta { get; init; }
+}