Add a native Cats Effect backend

[kciesielski]

Currently the only available native is CurlBackend. Let's add a native Cats Effect backend, this should be possible with current support for Scala Native in CE3.
For example, for Scala.js we have a FetchCatsBackend[F[_]: Async] extends AbstractFetchBackend. We might want something similar, based on the AbstractCurlBackend.

[Kamil-Lontkowski]

Since sttp targets Scala Native 0.5 in versions 3 and 4 (since 3.10.0 and 4.0.0-M13) and cats-effect supports only 0.4 it is blocked until Cats Effects publish version for Scala Native 0.5 (see typelevel/cats-effect#4076 for 0.5).