feat(tvix/store): implement reflection

This implements grpc.reflection.v1alpha.ServerReflection, and will make tools
like evans automatically discover available services, without having to
specify the path to the .proto files client-side.

It's behind a reflection feature flag, which is enabled by default.

Change-Id: Icbcb5eb05ceede5b9952e38a2ba72eaa6fa8a437
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7435
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/store/src/proto.rs

@@ -7,6 +7,12 @@ use prost::Message;
 
 tonic::include_proto!("tvix.store.v1");
 
+#[cfg(feature = "reflection")]
+/// Compiled file descriptors for implementing [gRPC
+/// reflection](https://github.com/grpc/grpc/blob/master/doc/server-reflection.md) with e.g.
+/// [`tonic_reflection`](https://docs.rs/tonic-reflection).
+pub const FILE_DESCRIPTOR_SET: &[u8] = tonic::include_file_descriptor_set!("tvix.store.v1");
+
 /// Errors that can occur during the validation of Directory messages.
 #[derive(Debug, PartialEq, Eq, Error)]
 pub enum ValidateDirectoryError {