Move from using asio::io_context to asio::executor for constructing async types

boost::asio has been moving away from using io_context for initialization to the more generally defined executor type, and in 1.70 the deprecated socket.get_io_context() is gone entirely. As boost::process remains unchanged in this regard, code that relied on being able to get the io_context from sockets and such as a means of initializing async_pipes no longer compiles. Consider moving towards accepting an executor instead.