Implement CAS support for BatchReadBlobs() method
Implement CAS support for BatchReadBlobs() method
Description
i. Implement ContentAddressableStorage.BatchReadBlobs()
ii. Now bgd cas download-file supports downloading a list of files, which will be requested from the server in a single batch.
The command receives an arbitrary list of space-separated 'digest path' pairs, and each request is allowed to fail individually.