feat(tvix/store/protos): add CA field

This adds support to represent the `CA` field found in some .narinfo
files. As `deriver`, it's also a subfield of the `narinfo` field.

Extending nix-compat with a more accessible data structure that can
take care of formatting, as well as writing validation functions in Rust
+ Golang, and integrating it into nar-bridge is something for a followup
CL.

Change-Id: I71e9c30957bcd03051a491aa54d7baac25b6dd2d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9795
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/cli/src/tvix_store_io.rs

@@ -344,8 +344,10 @@ async fn import_path_with_pathinfo(
             signatures: vec![],
             reference_names: vec![],
             deriver: None,
-            // TODO: narinfo for talosctl.src contains `CA: fixed:r:sha256:1x13j5hy75221bf6kz7cpgld9vgic6bqx07w5xjs4pxnksj6lxb6`
-            // do we need this anywhere?
+            ca: Some(tvix_store::proto::nar_info::Ca {
+                r#type: tvix_store::proto::nar_info::ca::Hash::NarSha256.into(),
+                digest: nar_sha256.to_vec().into(),
+            }),
         }),
     };