CAS Client: Use BatchUpdateBlobs

The Remote Execution API includes a BatchUpdateBlobs method for efficient upload of multiple small blobs. The CAS client currently does not use this. We should implement this to improve artifact push performance, especially across high latency network connections.

This depends on server-side support #676 (closed). The implementation should gracefully fallback to non-batch upload if the server does not support batch upload.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information