Clean up namespaces (#410)

No meaningful code changes, just lots of namespace shuffling to have fewer namespaces and to put things in better namespaces.

src/ModelContextProtocol/Protocol/RequestParams.cs

@@ -0,0 +1,22 @@
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Provides a base class for all request parameters.
+/// </summary>
+/// <remarks>
+/// See the <see href="https://github.com/modelcontextprotocol/specification/blob/main/schema/">schema</see> for details.
+/// </remarks>
+public abstract class RequestParams
+{
+    /// <summary>
+    /// Gets or sets metadata related to the request that provides additional protocol-level information.
+    /// </summary>
+    /// <remarks>
+    /// This can include progress tracking tokens and other protocol-specific properties
+    /// that are not part of the primary request parameters.
+    /// </remarks>
+    [JsonPropertyName("_meta")]
+    public RequestParamsMetadata? Meta { get; init; }
+}