Add logging support

A server can send structured logging messages to the client.
https://modelcontextprotocol.io/specification/2025-06-18/server/utilities/logging#logging

Logging was specified in the 2024-11-05 specification, but since it was not supported in ruby-sdk, I implemented it.
https://modelcontextprotocol.io/specification/2024-11-05/server/utilities/logging

I also made it possible to output a simple notification message in the examples.

lib/mcp/logging_message_notification.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require "json_rpc_handler"
+
+module MCP
+  class LoggingMessageNotification
+    LOG_LEVEL_SEVERITY = {
+      "debug" => 0,
+      "info" => 1,
+      "notice" => 2,
+      "warning" => 3,
+      "error" => 4,
+      "critical" => 5,
+      "alert" => 6,
+      "emergency" => 7,
+    }.freeze
+
+    def initialize(level:)
+      @level = level
+    end
+
+    def valid_level?
+      LOG_LEVEL_SEVERITY.keys.include?(@level)
+    end
+
+    def should_notify?(log_level)
+      return false unless LOG_LEVEL_SEVERITY.key?(log_level)
+
+      LOG_LEVEL_SEVERITY[log_level] >= LOG_LEVEL_SEVERITY[@level]
+    end
+  end
+end