Support Windows environment variables in NuGet.config files

[mikeyq6]

It would be immensely helpful to be able to have something like this in the NuGet.config file:
<add key="repositoryPath" value="%APPDATA%\NuGetPackages" />

That way you wouldn't have to hard-code a directory in a shared environment.

[yishaigalatzer]

Definitely something we can look into at some point. Will also be open to taking a pull request here.

Noting the cases to consider

1. Should we expand just values or also keys (probably just values)
2. Should expand the whole doc in one go (probably no, if yes we should consider the 32K limit)
3. Should this work on coreclr & Linux/Mac - Yes