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/src/dummy_blob_service.rs

@@ -0,0 +1,34 @@
+use tokio_stream::wrappers::ReceiverStream;
+
+use crate::proto::blob_service_server::BlobService;
+use crate::proto::BlobChunk;
+use crate::proto::BlobMeta;
+use crate::proto::PutBlobResponse;
+use crate::proto::ReadBlobRequest;
+use crate::proto::StatBlobRequest;
+use tonic::{Request, Response, Result, Status, Streaming};
+
+pub struct DummyBlobService {}
+
+#[tonic::async_trait]
+impl BlobService for DummyBlobService {
+    type ReadStream = ReceiverStream<Result<BlobChunk>>;
+
+    async fn stat(&self, _request: Request<StatBlobRequest>) -> Result<Response<BlobMeta>> {
+        Err(Status::unimplemented("not implemented"))
+    }
+
+    async fn read(
+        &self,
+        _request: Request<ReadBlobRequest>,
+    ) -> Result<Response<Self::ReadStream>, Status> {
+        Err(Status::unimplemented("not implemented"))
+    }
+
+    async fn put(
+        &self,
+        _request: Request<Streaming<BlobChunk>>,
+    ) -> Result<Response<PutBlobResponse>> {
+        Err(Status::unimplemented("not implemented"))
+    }
+}