Support NTCredentials in HttpConnectProxiedSocketAddress

Is your feature request related to a problem?

Yes - I'm the author of the Spark BigQuery connector which uses gRPC internally to communicate to the GCP APIs. A user is requesting to add support for NT Credentials for proxies.

Describe the solution you'd like

From the user's desription:

our proxy server only support NTCredentials to pass Authcations, we need to build a generic way to support Username, NT, Kerberos Poxy Auth. for our code we just need to replace usernamepasswordCredentials(user,pass) by NTCredentials(user,pass,null, region)

Describe alternatives you've considered

It seems there are none

Additional context