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/Cargo.lock

@@ -1987,6 +1987,21 @@ dependencies = [
  "syn 1.0.103",
 ]
 
+[[package]]
+name = "tonic-reflection"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0455f730d540a1484bffc3c55c94100b18a662597b982c2e9073f2c55c602616"
+dependencies = [
+ "bytes",
+ "prost",
+ "prost-types",
+ "tokio",
+ "tokio-stream",
+ "tonic",
+ "tonic-build",
+]
+
 [[package]]
 name = "tower"
 version = "0.4.13"
@@ -2156,6 +2171,7 @@ dependencies = [
  "tokio-stream",
  "tonic",
  "tonic-build",
+ "tonic-reflection",
 ]
 
 [[package]]