Support Digest Authentication for Proxies

I am trying to figure out if the BigTable HBase client supports digest authentication. The google bigtable team asked me to check here.

### What version of gRPC are you using?
Whatever the latest bigtable hbase client is using.

### What did you expect to see?
digest authentication.

The issue names the BigTable HBase client and its gRPC context but mentions no repository files, tests, or entry points. Start by locating existing proxy and authentication support in grpc-java and clarify the expected digest-authentication behavior; done requires an agreed implementation scope and verification path.

Scritto dal modello di indicizzazione a partire dal testo della issue.