feat(nix-daemon): Implement client handler.

This change includes only the basic nix handshake protocol handling and
sets up a client session. The only supported operation at this point is
SetOptions.

Additional operations will be implemented in subsequent cls.

Change-Id: I3eccd9e0ceb270c3865929543c702f1491768852
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12743
Autosubmit: Vladimir Kryachko <v.kryachko@gmail.com>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Reviewed-by: edef <edef@edef.eu>
Reviewed-by: Brian Olsen <me@griff.name>

tvix/nix-compat/src/nix_daemon/mod.rs

@@ -1 +1,8 @@
+pub mod handler;
+pub mod types;
 pub mod worker_protocol;
+
+/// Represents all possible operations over the nix-daemon protocol.
+pub trait NixDaemonIO {
+    // TODO add methods to it.
+}