Use sbt's multi-jvm instead of our manual implementation?

As a result of the discussion at https://github.com/apache/incubator-pekko-http/pull/297#issuecomment-1669011499 I just found out that there is already a [sbt-multi-jvm](https://github.com/sbt/sbt-multi-jvm) plugin that appears to be identical to ours and was originally upstream by Akka/Akka community, presumably for Akka to eventually remove the multi jvm setup in their builds which we inherited.

I think we should seriously consider upstreaming any necessary changes and then using the plugin, especially considering that we now have duplicated code between pekko and pekko-http.

@jrudolph @pjfanning @He-Pin @raboof wdyt ?