Fs2: Add support for unix sockets

[Taig]

Following this discussion on Gitter it appears feasible to add support for unix sockets to fs2 based on jnr-unixsockets.

As a use case example this could benefit a library like tpolecat/skunk to talk to a postgres database via unix sockets as it is done on Google Cloud (Run).

[mpilquist]

I think @ChristopherDavenport implemented this recently. I'd be happy to incorporate it in to fs2-io if Chris wants but knowing Chris, it's already available somewhere. :)

[ChristopherDavenport]

https://github.com/ChristopherDavenport/whale-tail/blob/master/core/src/main/scala/io/chrisdavenport/whaletail/UnixSocket.scala - Works like a charm. Was going to move it into a library of its own.

[ChristopherDavenport]

Always happy to get code off my hands though if you want it.

[ChristopherDavenport]

Otherwise I was going to move it into something like fs2-unixsocket or something to that effect here shortly. Was finishing ember to prove all my concepts work. But its pretty solid at communicating with the docker daemon already.

[ChristopherDavenport]

Alright, going to contribute this as a new module to fs2 as long as everyone is happy.

[Taig]

I don't have an immediate use for this, I was just investigating the topic and wanted to document the gitter discussion in the issues to provide a better point of reference for other people searching for this. Knowing that I can find a solution for this in the whaletail repo is totally sufficient for me, but having a proper fs2 module would certainly be a convenient solution.

[keynmol]

Probably a very rare use-case, but the jnr-unixsocket library relies on https://github.com/jnr/jnr-ffi which makes use of dynamic class loading for platform-specific stuff, which breaks immediately when building with graalvm native-image (no fallback)

Also there's JEP effort to provide a generic solution in the JDK itself: https://openjdk.java.net/jeps/380 which hopefully won't rely on dynamic class loading, so should be more portable.