Add awaitable to forward declarations

[SmartAI]

Summary

• Add awaitable to asio/fwd.hpp when coroutine support is available.
• Share one guarded declaration with awaitable.hpp, keeping the default executor argument in a single declaration.
• Extend the forward-header test to declare awaitable<void> before the full header is included.

Motivation

asio/fwd.hpp cannot currently declare an API that returns asio::awaitable<T> without including the complete coroutine implementation.
This fills that gap while preserving the existing default executor and versioned namespace behavior.

Testing

• GCC 16.2: official Autotools targets for fwd_first and fwd_last.
• GCC 15.2 and Clang 21.1: fwd_first and fwd_last with versioned namespace, TS executor default, their combination, and ASIO_DISABLE_CO_AWAIT.
• C++11 compile checks for the non-coroutine path.
• perl ./boostify.pl --includes-only and generated Boost.Asio forward-header probes with GCC and Clang.