Use autoload to defer loading of unused subsystems

## Motivation and Context

`lib/mcp.rb` currently declares all `require_relative` statements in one place.
As a result, Server and Client are both loaded, the stdio and Streamable HTTP
transports are both loaded, and Tool, Prompt, and Resource are all loaded,
regardless of which components are actually used.

These components are selected by the user depending on their use case,
so loading everything upfront is unnecessary.

Using `autoload` for selected constants defers loading until the constant
is actually referenced, so only the subsystems required by the user are loaded.

## How Has This Been Tested?

All existing tests pass unchanged.

## Breaking Change

None. This change only defers loading of unused components.

test/mcp/instrumentation_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "test_helper"
+require "mcp/instrumentation"
 
 module MCP
   class InstrumentationTest < ActiveSupport::TestCase

test/mcp/methods_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "test_helper"
+require "mcp/methods"
 
 module MCP
   class MethodsTest < ActiveSupport::TestCase