Add NuGet config

[manodasanW]

Adding NuGet.config with public feed that upstreams to NuGet.org per guidance.

[sylveon]

What about external users? Since this is in the root of the repo, their Visual Studio will end up picking up this feed as well - is this intentional?

[manodasanW]

@sylveon the feed is a public externally accessible feed, so external users should have access to it. Let me know if you don't and I can look into that.

[DefaultRyan]

It's also worth noting that all the CI tests passed, and they don't have any priveleged access to internal feeds - the CI tests can only access public feeds just like external users.

[sylveon]

Ah, fair enough then

[walbourn]

Note that another solution would be to not have a nuget.config at all and use in YAML:

  - task: NuGetCommand@2
    displayName: NuGet restore
    inputs:
      solution: cppwinrt.sln
      feedRestore: $(GUID_FEED)
      includeNuGetOrg: false

The GUID_FEED is set as a variable so it only gets used by ADO pipelines and not everyone else. GitHub Actions can just use nuget.org directly.

For GitHub Actions, use:

 uses: nuget/setup-nuget@v2