Fix all async initiating functions with use_awaitable:

Add trait:
    is_completion_token_for

Fixes for
    buffered_read_stream

Add tests for
    basic_stream
    buffered_read_stream
    flat_stream
    http (all)
    websocket (all)

resolves #1920

test/beast/websocket/handshake.cpp

@@ -18,6 +18,9 @@
 #include <boost/asio/io_context.hpp>
 #include <boost/asio/strand.hpp>
 #include <thread>
+#if BOOST_ASIO_HAS_CO_AWAIT
+#include <boost/asio/use_awaitable.hpp>
+#endif
 
 namespace boost {
 namespace beast {
@@ -704,6 +707,23 @@ public:
         }
     }
 
+#if BOOST_ASIO_HAS_CO_AWAIT
+    void testAwaitableCompiles(
+        stream<test::stream>& s,
+        std::string host,
+        std::string port,
+        response_type& resp)
+    {
+        static_assert(std::is_same_v<
+            net::awaitable<void>, decltype(
+            s.async_handshake(host, port, net::use_awaitable))>);
+
+        static_assert(std::is_same_v<
+            net::awaitable<void>, decltype(
+            s.async_handshake(resp, host, port, net::use_awaitable))>);
+    }
+#endif
+
     void
     run() override
     {
@@ -714,6 +734,9 @@ public:
         testMoveOnly();
         testAsync();
         testIssue1460();
+#if BOOST_ASIO_HAS_CO_AWAIT
+        boost::ignore_unused(&handshake_test::testAwaitableCompiles);
+#endif
     }
 };