Early PR: Support ntlm based proxy in tunnel agent.

[omeshp]

Raising an early PR to get feedback regarding supporting NTLM based proxies in tunnel agent.

[omeshp]

@koichik Would you be interested in this feature addition to tunnel agent? Raised this early PR to get your thoughts on these changes?

We use tunnel agent as part of Artifact Engine (https://github.com/Microsoft/azure-pipelines-extensions/tree/master/Extensions/ArtifactEngine) and we saw a lot of people running into issues using it since they are on NTLM proxies.