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/store/src/bin/tvix-store.rs

@@ -21,6 +21,7 @@ use tvix_castore::proto::GRPCBlobServiceWrapper;
 use tvix_castore::proto::GRPCDirectoryServiceWrapper;
 use tvix_castore::proto::NamedNode;
 use tvix_store::pathinfoservice;
+use tvix_store::proto::nar_info;
 use tvix_store::proto::path_info_service_server::PathInfoServiceServer;
 use tvix_store::proto::GRPCPathInfoServiceWrapper;
 use tvix_store::proto::NarInfo;
@@ -303,6 +304,10 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
                                 signatures: vec![],
                                 reference_names: vec![],
                                 deriver: None,
+                                ca: Some(nar_info::Ca {
+                                    r#type: tvix_store::proto::nar_info::ca::Hash::NarSha256.into(),
+                                    digest: nar_sha256.to_vec().into(),
+                                }),
                             }),
                         };