fix: Data race in new webserver (#1926)

There was a data race inside `CoroutineGroup` because internal timer was
used from multiple threads in the methods `asyncWait()` and
`onCoroutineComplete()`. Changing `registerForeign()` to spawn to the
same `yield_context` fixes the problem because now the timer is accessed
only from the same coroutine which has an internal strand.

During debugging I also added websocket support for `request_gun` tool.

src/util/CoroutineGroup.hpp

@@ -73,10 +73,11 @@ public:
      * @note A foreign coroutine is still counted as a child one, i.e. calling this method increases the size of the
      * group.
      *
+     * @param yield The yield context owning the coroutine group.
      * @return A callback to call on foreign coroutine completes or std::nullopt if the group is already full.
      */
     std::optional<std::function<void()>>
-    registerForeign();
+    registerForeign(boost::asio::yield_context yield);
 
     /**
      * @brief Wait for all the coroutines in the group to finish