Add default completion token for asio::deferred_async_operation

[TCPLK]

Asynchronous operation composed using asio::deferred and the pipe operator still require asio::deferred to be passed explicitly when invoked

auto timeout_read = socket.async_read_some(asio::buffer(buf)) | asio::cancel_after(timer, 1s);

auto read_size = co_await timeout_read(asio::deferred);  // ok
auto read_size = co_await timeout_read();  // compile failed

Adding a default completion token to asio::deferred_async_operation to align with other asio asynchronous functions would improve usability, especially when using coroutines

[TCPLK]

It would be great if default_completion_token_t could be used for this code

asio/include/asio/deferred.hpp

Lines 348 to 358 in bd500f0

	template <ASIO_COMPLETION_TOKEN_FOR(Signature) CompletionToken>
	auto operator()(CompletionToken&& token) &&
	-> decltype(
	this->invoke_helper(
	static_cast<CompletionToken&&>(token),
	detail::index_sequence_for<InitArgs...>()))
	{
	return this->invoke_helper(
	static_cast<CompletionToken&&>(token),
	detail::index_sequence_for<InitArgs...>());
	}

like this:

/// Initiate the asynchronous operation using the supplied completion token.
template <ASIO_COMPLETION_TOKEN_FOR(Signature) CompletionToken = default_completion_token_t<...>>
auto operator()(CompletionToken&& token = default_completion_token_t<...>()) &&
{
    ...
}