Allow passing proxy settings to agent configuration in Install-VSTSAgent

[japj]

In case you are running the agent behind a proxy server, you need to pass additional options to the agent configure script as described by https://docs.microsoft.com/en-us/vsts/build-release/actions/agents/proxy?view=vsts

From looking at the Install-VSTSAgent code this does not seem to be supported yet?

[jwittner]

That'd be a great and simple enhancement! Very open to contributions if you'd like to add it! =) Let me know.

[temelj]

Any plans to include this in the future or do we have to contribute?

(Yes: Who uses on-prem build agents, especially behind a proxy :( )

[jwittner]

A contribution would be most welcome!