feat(tvix/castore): extend blobstore protos for verified streaming

This pdates the proto docstrings a bit, especially w.r.t. verified
streaming.
It also adds send_chunks, send_bao fields to StatBlobRequest (renamed
from BlobMeta)

Change-Id: I590cc8646d86b73bca9f38a9b6d9ea15e4df5cb6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9951
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/castore/src/blobservice/grpc.rs

@@ -56,6 +56,7 @@ impl BlobService for GRPCBlobService {
         let resp = grpc_client
             .stat(proto::StatBlobRequest {
                 digest: digest.clone().into(),
+                ..Default::default()
             })
             .await;