feat(tvix/nar-bridge): init

This adds an implementation of nar-bridge in Rust.
Currently, only the GET parts are implemented.

Contrary to the Go variant, this doesn't try to keep a mapping from nar
hashes to root node in memory, it simply encodes the root node itself
(stripped by its basename) into the URL.

This pulls in a more recent version of axum than what we use in
tonic, causing two versions of http and hyper, however dealing with
`Body::from_stream` in axum 0.6 is much more annoying, and
https://github.com/hyperium/tonic/pull/1740 suggests this will be fixed
soon.

Change-Id: Ia4c2dbda7cd3fdbe47a75f3e33544d19eac6e44e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11898
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Brian Olsen <me@griff.name>
Tested-by: BuildkiteCI

tvix/Cargo.toml

@@ -25,6 +25,7 @@ members = [
   "eval",
   "eval/builtin-macros",
   "glue",
+  "nar-bridge",
   "nix-compat",
   "serde",
   "store",