Add support for Corporate Proxys that utilize Kerberos authentication

**Is your feature request related to a problem? Please describe.**

Az CLI does not support communication with a corporate Proxy which utilizes Kerberos tickets for authentication. The interesting behavior is that az cli is able to authenticate (even though it does not support Kerberos tickets for Proxy auth) to the corporate proxy and intermittently function even though username and password are not specified as part of `HTTP_PROXY` and/or `HTTPS_PROXY` env variables.

When these intermittent failures occur we performed the following:

* Checked the validity/expiration of the Kerberos tickets by running `klist` and verified these tickets were not expired
* Learnt that opening up Microsoft Edge and visiting a site on the Internet would consistently be it temporarily remidiated this issue until next time

**Describe the solution you'd like**

Az CLI should support communication with a corporate Proxy which utilizies Kerberos authentication.

We had this discussion with Microsoft and got the answer that the underlying open source Python library being used to faciliate network communication does not support the above mentioned behavior. Our response is that **Microsoft being an enterprise serving organization should contribute back to the open source Python library and add support for Kerberos authentication for proxy communication as most of their corporate customers operate behind a proxy.**

**Describe alternatives you've considered**

Alternative is to in plaintext specify the username and password along with the proxy URL in the env vars: `HTTP_PROXY` and `HTTPS_PROXY` which is insecure and not best practice.

**Additional context**

Intermittently receive 407 Proxy Authentication required errors. **Opening Microsoft Edge and visiting a site on Internet resolved this issue temporarliy. What is unclear to us why is that the case?**