Improve NuGet API Parity (#21291)

[ScionOfDesign]

Summary

Fixes #21291, allowing icons and other missing attributes to appear for NuGet packages from inside Visual Studio like they do with GitHub Nuget packages.

Adds additional NuGet package information, particularly IconURL, to bring the Gitea NuGet API more in-line with GitHub's NuGet API.

• Adds the following .nuspec values to the NuGet query responses:
  • copyright
  • iconUrl
    • Adding this allows Visual Studio to display and Icon for the package.
  • isPrerelease
  • language
  • licenseUrl
    • The license now appears in Visual Studio.
  • summary
    • GitHub responds with this value, but it is not supported by MSBuild. It is part of the NuGet Api Specification, though.
  • tags
    • Tags now appear in Visual Studio.
  • dependencyGroups
    • Now appears in all requests for a package like with GitHub.
  • totalDownloads
    • Now calculated while gathering the different package versions.
• Re-orders the values in the response to more closely align with GitHub.

References

Octopus Samples - Examples of NuGet packages being pushed to GitHub.
When querying for packages, Visual Studio makes a request to a route like this one:
https://nuget.pkg.github.com/OctopusSamples/OctopusSamples.accept-message/index.json

Gitea was missing the properties that GitHub responds with.
Both Gitea and Github should now look like the following with the attached package, for example. (Rename it to .nupkg)

It should be noted that GitHub also has a boolean property called verified, but this is not part of the NuGet specification.

Please let me know if anything else should be changed.

[wxiaoguang]

Can we have some tests? Existing tests are in api_packages_nuget_test.go

[ScionOfDesign]

Alright, let me look into updating the tests.

[ScionOfDesign]

I have updated the tests to check for the additional values present in the API response.

[wxiaoguang]

Thank you very much!