Add support to store proxy credentials in Windows Credentials Manager

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

Currently, az cli does not support Kerberos ticket-based authenticaiton to a Proxy. The only documented way to have az cli authenticate out through a corporate proxy is to in **plaintext hardcode the username and password along with the proxy URL in the `HTTP_PROXY` and `HTTPS_PROXY` env variables**. This is insecure and not best practice which can lead to credential leaks.

**Describe the solution you'd like**

Add support to **store encrypted and read proxy credentials** (username and password) from **Windows Credentials Manager** when using az cli.

**Describe alternatives you've considered**

Alternative, credentials in plaintext in env variables. Not acceptable.

**Additional context**

N/A