default-completion tokens

[klemens-morgenstern]

With boost 1.86, there is a global default completion token, so this library should add support.

Changing this

template<typename CompletionToken> 
auto async_foo( ..., CompletionToken && token)

to

template<typename CompletionToken = typename ::boost::asio::default_completion_token<executor_type>::type> 
auto async_foo(..., CompletionToken && token = typename ::boost::asio::default_completion_token<executor_type>::type())

should do that and will allow code like this:

co_await async_foo(...); // not token needed, default is co_await-able 

[ksimicevic]

Resolved with 7bc1ccf072edd1c87e6c343383c27606d83e9be3.