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/store/Cargo.toml

@@ -13,6 +13,8 @@ bytes = { workspace = true }
 clap = { workspace = true, features = ["derive", "env"] }
 count-write = { workspace = true }
 data-encoding = { workspace = true }
+ed25519 = { workspace = true }
+ed25519-dalek = { workspace = true }
 futures = { workspace = true }
 lazy_static = { workspace = true }
 nix-compat = { path = "../nix-compat", features = ["async"] }