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/icon.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+module MCP
+  class Icon
+    SUPPORTED_THEMES = ["light", "dark"]
+
+    attr_reader :mime_type, :sizes, :src, :theme
+
+    def initialize(mime_type: nil, sizes: nil, src: nil, theme: nil)
+      raise ArgumentError, 'The value of theme must specify "light" or "dark".' if theme && !SUPPORTED_THEMES.include?(theme)
+
+      @mime_type = mime_type
+      @sizes = sizes
+      @src = src
+      @theme = theme
+    end
+
+    def to_h
+      { mimeType: mime_type, sizes: sizes, src: src, theme: theme }.compact
+    end
+  end
+end