NuGet config command doesn't check if the key is valid

[heng-liu]

NuGet.exe config -Set doesn't check if the key is valid.
So if we run NuGet.exe config -Set ABC=abc, the following will be added into NuGet.Config file, even though ABC is not a valid key from config-section

  <config>
    <add key="ABC" value="abc" />
  </config>

Since we're writing a spec of adding new dotnet nuget config command, we will need to discuss if we want to keep dotnet nuget config command consistent with NuGet.exe, or change the NuGet.exe to check the key.