Add direct authentication support for the upstream proxy agent (HTTP Basic, NTLM, Negotiate)

[Andreybest]

Is your feature request related to a problem? Please describe.
Follow-up to #1458 (upstream proxy support).
The initial upstream proxy implementation does not include direct authentication support in the proxy agent.

Describe the solution you'd like
Add support for the three most common upstream proxy authentication methods:

• HTTP Basic — credentials sent in the Proxy-Authorization header.
• NTLM — Windows challenge/response authentication.
• Negotiate — the SPNEGO/Negotiate protocol, which detects whether NTLM or Kerberos is supported and then performs whichever the server negotiates.

Describe alternatives you've considered
None

Additional context
PR #1458