NuGet v2 $metadata endpoint

What does this MR do and why?

To be able to install Nuget packages using NuGet v2 feed, the $metadata endpoint should be supported:

• GET /api/v4/projects/<project_id>/packages/nuget/v2/$metadata

This endpoint should give information about the 'feeds exposed by the data service. It's not documented anymore by NuGet but it's publicly available at https://www.nuget.org/api/v2/$metadata, so I followed NuGet implementation and limited its response to suit our needs.

Screenshots or screen recordings

How to set up and validate locally

The endpoint can be directly checked in the browser:

http://gdk.test:3000/api/v4/projects/<project_id>/packages/nuget/v2/$metadata

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #416405 (closed)