chore(tvix/nix-daemon): Implement STDERR_READ protocol

When sending nars over the wire to the nix-daemon, nix protocol versions
1.21 to 1.23 use this framing protocol.

This change implements an AsyncRead for this protocol, to be used in
AddToStoreNar and any other operations when necessary.

Change-Id: I571f1adbb2343c14c98503d1a2c12eea4c783ec9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12813
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

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

@@ -8,6 +8,7 @@ use types::{QueryValidPaths, UnkeyedValidPathInfo};
 
 use crate::store_path::StorePath;
 
+pub mod framing;
 pub mod handler;
 pub mod types;