Support NTCredentials in HttpConnectProxiedSocketAddress

### Is your feature request related to a problem?
Yes - I'm the author of the [Spark BigQuery connector](https://github.com/GoogleCloudDataproc/spark-bigquery-connector) which uses gRPC internally to communicate to the GCP APIs. A user is [requesting](https://github.com/GoogleCloudDataproc/spark-bigquery-connector/issues/1071) 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