feat(tvix/store): initial dummy implementation

This replaces the hello world example from tvix-store with an actual
gRPC endpoint, implementing all of BlobService, DirectoryService and
PathInfoService.

All RPC methods currently respond with the unimplemented gRPC status.

Co-Authored-By: Márton Boros <martonboros@gmail.com>
Change-Id: Ieba333cca44dc1e3f2ffbe676ba7a99e672b9bfb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7664
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/store/Cargo.toml

@@ -8,8 +8,11 @@ anyhow = "1.0.68"
 blake3 = { version = "1.3.1", features = ["rayon", "std"] }
 data-encoding = "2.3.3"
 lazy_static = "1.4.0"
+clap = { version = "4.0", features = ["derive", "env"] }
 prost = "0.11.2"
 thiserror = "1.0.38"
+tokio = { version = "1.23.0", features = ["rt-multi-thread"] }
+tokio-stream = "0.1.11"
 tonic = "0.8.2"
 
 [build-dependencies]