Add a setting for allowing/disallowing duplicate NuGet package uploads

Problem to solve

When using the GitLab Package Registry to publish NuGet packages, a duplicate package name/version can be uploaded. This may be great for snapshots, but you may want your releases to be immutable.

Proposal

Add a setting that will allow you to define (at the group level) whether or not duplicate NuGet packages are allowed.

Further details

Please make sure that the dotnet cli command line option "--skip-duplicate" works, which means you should return the same error that nuget.org returns, when a duplicate is uploaded.

• dotnet nuget push documentation: https://docs.microsoft.com/en-us/dotnet/core/tools/dotnet-nuget-push
• As per #262081 (comment 587195702), we'll also need to consider behavior for symbol packages which by default allow duplicates.
• As per #336225 (comment 633958497) we will consider adding a setting to overwrite existing packages