Instance-level NuGet endpoint

What does this MR do?

This MR implements an endpoint at the Instance-level that supports pulling NuGet packages. Closes #36425

What does this MR not do?

This MR does not support publishing NuGet packages to the Instance-level URL.

Screenshots

PS C:\Users\firelizzard\Desktop\Test> dotnet add package namespace3:project3:NugetPackage2
  Determining projects to restore...
  Writing C:\Users\firelizzard\AppData\Local\Temp\tmp316A.tmp
info : Adding PackageReference for package 'namespace3:project3:NugetPackage2' into project 'C:\Users\firelizzard\Desktop\Test\Test.csproj'.
info : Restoring packages for C:\Users\firelizzard\Desktop\Test\Test.csproj...
info :   GET http://gitlab.example.com/api/v4/packages/nuget/download/namespace3:project3:nugetpackage2/index.json
info :   GET https://api.nuget.org/v3-flatcontainer/namespace3:project3:nugetpackage2/index.json
info :   NotFound https://api.nuget.org/v3-flatcontainer/namespace3:project3:nugetpackage2/index.json 96ms
info :   OK http://gitlab.example.com/api/v4/packages/nuget/download/namespace3:project3:nugetpackage2/index.json 487ms
info :   GET http://gitlab.example.com/api/v4/packages/nuget/download/namespace3:project3:nugetpackage2/1.0.2/namespace3:project3:nugetpackage2.1.0.2.nupkg
info :   OK http://gitlab.example.com/api/v4/packages/nuget/download/namespace3:project3:nugetpackage2/1.0.2/namespace3:project3:nugetpackage2.1.0.2.nupkg 556ms

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team