Add the git "repository url" nuspec field as an asset attribute for Nuget repositories

As an auditor looking at artifacts via the UI, it would be useful if I could see what source code repository (e.g., the git repository) was used to build a nupkg artifact that is in the Nexus Nuget repository.

This piece of metadata is typically stored in the "RepositoryUrl" field of a csproj file and appears in the "repository" element of the nuspec file that is a part of the nupkg that is pushed into Nexus.

Currently it looks like Nexus is already displaying metadata from the nuspec file as asset attributes (e.g., the description, authors, require license, etc).  It should be an easy win to add the git url as well.