feat(tvix/store): Add a signing PathInfoService

- Add a new PathInfoService implementation that wraps transparently
around another except that it dynamically signs all the incoming
path-infos with the provided signer.

- Add a ServiceBuilder for this PathInfoService that provides a
SigningPathInfoService with a keyfile signer

Change-Id: I845ddfdf01d14c503c796b2b80c720dab98be091
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12032
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: sinavir <tvix@sinavir.fr>
Tested-by: BuildkiteCI

tvix/Cargo.nix

@@ -15572,6 +15572,14 @@ rec {
             name = "data-encoding";
             packageId = "data-encoding";
           }
+          {
+            name = "ed25519";
+            packageId = "ed25519";
+          }
+          {
+            name = "ed25519-dalek";
+            packageId = "ed25519-dalek";
+          }
           {
             name = "futures";
             packageId = "futures";