NuGet - API Skeleton + The Service Index + authentication + feature flag

Create the initial file structure for NuGet API endpoints and implement the endpoints needed for authentication behind a feature flag.

This will implement the NuGet Service Index (see investigation issue for details) which will initially return the list of API endpoints: example response.

All NuGet API routes from all of the other services will be added in this step with an empty 404 response.

Technical notes

• Authentication is done through http basic header but the feed has to be under https for Nuget to send the header along with the requests. (Use ngrok.io locally to emulate an https server with basic auth)
• Conan implementation has the http basic auth: https://gitlab.com/gitlab-org/gitlab/blob/master/ee%2Flib%2Fapi%2Fconan_packages.rb#L354-359. Similar work can be used here or better, centralize code for both APIs.

Related Issues:

Investigation: #33590 (closed) NuGet Parent issue: #20050 (closed)

This is step 1 of 7 in implementing the NuGet API MVC:

1. #36496 (closed) - API Skeleton + The Service Index + authentication
2. gitlab-workhorse#235 (closed) - Workhorse upload route
3. #36499 (closed) - Push/Delete Service
4. #36502 (closed) - Metadata extraction job, DB changes, and dependencies
5. #36504 (closed) - Package Metadata Service
6. #36505 (closed) - Search Service
7. #36506 (closed) - Package Content Service