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/Cargo.lock

@@ -2300,6 +2300,7 @@ dependencies = [
  "nix-compat-derive",
  "nom",
  "num-traits",
+ "num_enum",
  "pin-project-lite",
  "pretty_assertions",
  "proptest",
@@ -2423,6 +2424,27 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "num_enum"
+version = "0.7.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4e613fc340b2220f734a8595782c551f1250e969d87d3be1ae0579e8d4065179"
+dependencies = [
+ "num_enum_derive",
+]
+
+[[package]]
+name = "num_enum_derive"
+version = "0.7.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "af1844ef2428cc3e1cb900be36181049ef3d3193c63e43026cfe202983b27a56"
+dependencies = [
+ "proc-macro-crate",
+ "proc-macro2",
+ "quote",
+ "syn 2.0.79",
+]
+
 [[package]]
 name = "number_prefix"
 version = "0.4.0"
@@ -2833,6 +2855,15 @@ dependencies = [
  "syn 2.0.79",
 ]
 
+[[package]]
+name = "proc-macro-crate"
+version = "3.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8ecf48c7ca261d60b74ab1a7b20da18bede46776b2e55535cb958eb595c5fa7b"
+dependencies = [
+ "toml_edit 0.22.22",
+]
+
 [[package]]
 name = "proc-macro-error-attr2"
 version = "2.0.0"