feat(gs/xanthous): Add SSH server application

Add the start of an ssh-compatible server application for playing
Xanthous over ssh. The idea here is that the user can ssh with whatever
username they like (defaulting obviously to their login username) and
then that gets passed as the --name flag to xanthous as they play.

Change-Id: I39eec288ac8e22a165e859479888d78158a58818
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3806
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

users/grfn/xanthous/server/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "xanthous-server"
+version = "0.1.0"
+edition = "2018"
+
+[dependencies]
+clap = "3.0.0-beta.5"
+color-eyre = "0.5.11"
+eyre = "0.6.5"
+thrussh = "0.33.5"
+thrussh-keys = "0.21.0"
+tracing = "0.1.29"
+tracing-subscriber = "0.2.25"
+futures = "0.3.17"
+libc = "0.2.103"
+nix = "0.23.0"
+
+# Pins for rust 1.55 (2018 edition) until we have 1.56 in nixpkgs-unstable
+pbkdf2 = "<0.9"
+base64ct = "<1.2"
+
+[dependencies.tokio]
+version = "1.13"
+features = ["rt", "rt-multi-thread", "macros", "net", "process", "fs", "signal"]