Add Proxy Support for WikiPDFExport

[Sebbl22]

We use Azure DevOps Server 2019 on-premise. Our build agent can access the web, but is behind a HTTP proxy. Downloading azuredevops-export-wiki.exe fails, because the connection is refused (##[error]connect ECONNREFUSED 140.82.121.6:443).
The NPM-package download-github-release, which is used to download azuredevops-export-wiki.exe, does not honor the agent proxy settings.

The WikiPDFExport extension should use agent proxy settings, when they are configured correctly as described here.
The proxy values can be retrieved as described here. Unfortunately I could not find any documentation on proxy support / how to pass proxy parameters for download-github-release, so it may need to be replaced.

Alternatively, the path to an already downloaded azuredevops-export-wiki.exe could be specified.

[rfennell]

The quickest solution is to add the option to provide a path to a previously download azuredevops-export-wiki.exe.

I will have a look at honouring the agent proxy settings, but I am not sure when I will have time to investigate it fully.

[rfennell]

The 1.3.8 release provides a way to override the path to the exe, so a workaround for now

[Sebbl22]

Thank you

[rfennell]

I cannot find any Node libraries that handle GitHub release downloads and allows proxy settings to be set.

I don't have the time to investigate writing such as client for this edge case at present, especially as there is now a workaround. Hence, I have marked this issue as a help wanted issue. So best to assume no fix in the near future