Support icons parameter

This PR adds support for the optional `icons` parameter in prompt, resource,
resource template, and server info.

The specifications for `icons` are as follows.
https://modelcontextprotocol.io/specification/2025-11-25/schema#icon

Resolves https://github.com/modelcontextprotocol/ruby-sdk/issues/127.

lib/mcp/resource_template.rb

@@ -2,13 +2,14 @@
 
 module MCP
   class ResourceTemplate
-    attr_reader :uri_template, :name, :title, :description, :mime_type
+    attr_reader :uri_template, :name, :title, :description, :icons, :mime_type
 
-    def initialize(uri_template:, name:, title: nil, description: nil, mime_type: nil)
+    def initialize(uri_template:, name:, title: nil, description: nil, icons: [], mime_type: nil)
       @uri_template = uri_template
       @name = name
       @title = title
       @description = description
+      @icons = icons
       @mime_type = mime_type
     end
 
@@ -18,6 +19,7 @@ module MCP
         name: name,
         title: title,
         description: description,
+        icons: icons.map(&:to_h),
         mimeType: mime_type,
       }.compact
     end