Add an ability to set a custom NuGet source

[ChushkinAE]

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

Our CI pipeline doesn't have access to a standard Microsoft NuGets source due to security policy. We use our Artifactory as an origin of packages. We can't change a source in configuration files, because it has been written as a code constant "NuGetOnlineRepository" (https://github.com/OData/ODataConnectedService/blob/ed1a176e0f7d38d9b9519fc27a8c5c21b461da8c/src/Microsoft.OData.CodeGen/Common/Constants.cs#L42C85-L42C85).

Describe the solution you'd like

We want to have an ability to configure an origin of NuGets.

Describe alternatives you've considered

If we don't find support, we will create a fork with our constant. It's not the best idea, because we do not use GitHub as a code storage, and we will face with update's problems.