Cannot provide auth using `dnx`

### Is your feature request related to a problem? Please describe.

I am trying to do run a tool using dnx, where the tool is in github nuget feed, but `dnx` lacks an command argument for auth.

### Describe the solution you'd like
```diff
dnx Company.InternalTool.PackageId \
--source "https://nuget.pkg.github.com/mycompany/index.json" \
+++ --apikey "GITHUB_PAT_HERE"
```

### Additional context
There is no `--apikey` parameter allowing me to run `dnx` against a private feed. It is possible to create an upfront nuget.config-file and use a credential provider, but that's not what I'm after. `dnx` should work one-off, easy to run without pre-existing setup. Without an argument to pass auth, this is not currently possible.

A maintainer can push nugets, but a client cannot fetch them using the same mechanism:

```
✅ dotnet nuget push mypackage.nupkg --source "https://nuget.pkg.github.com/mycompany/index.json" --api-key
❌ dnx mypackage --source "https://nuget.pkg.github.com/mycompany/index.json" --api-key
```