Currently to use http4s-curl we use CurlApp which replaces the IORuntime with a CurlRuntime. This means that it is not possible to integrate http4s-curl with FS2-based applications (Ember, Skunk, Lepus) and other native libraries (e.g. SNUnit).

In Cats Effect 3.6 the default runtime offers file descriptor I/O polling that works with all of these libraries. The idea would be to use IOApp as usual and then to construct a Resource[IO, Client[IO]] with a CurlClientBuilder (similar to Ember).

Here's a rough sketch for how to implement a Client[IO] using file descriptor polling:

1. Create a MapRef[IO, FileDescriptor, (Fiber, Fiber)] to keep track of a dedicated fiber for polling each relevant file descriptor for each type of interest (read interest vs write interest).

2. Configure the CURLMOPT_SOCKETFUNCTION to get notified about when cURL needs to start/stop monitoring sockets for read/writes. Use these callbacks to start/cancel fibers in the MapRef.
   https://curl.se/libcurl/c/CURLMOPT_SOCKETFUNCTION.html

3. To add a new socket you should register the file descriptor.

4. Then, start a fiber looping on read-readiness and another looping on write-readiness. Whenever the socket is ready, the curl_multi_socket_action should be invoked.
   https://curl.se/libcurl/c/curl_multi_socket_action.html

5. A callback for CURLMOPT_TIMERFUNCTION should also be registered. It should start a fiber that sleeps the requested time and then calls curl_multi_socket_action.
   https://curl.se/libcurl/c/CURLMOPT_TIMERFUNCTION.html