Support Unity UPM packages

The unity ecosystem is slowly moving towards package management with its [UPM (Unity Package Manager)](https://docs.unity3d.com/Manual/Packages.html) format. Even tough the asset store does not yet fully support them, git repositories are supported, and there are also community driven UPM repositories which are getting more and more tracksion (for example https://openupm.com). Also NuGet packages can be installed via the [Xoofx UnityNuGet](https://github.com/xoofx/UnityNuGet/) UPM server project.

Getting dependabot support would be a huge plus. On the technical side I suppose it would not require too much work, since UPM is mostly just a rebranding of NPM.

Start with the linked Unity Package Manager documentation, then compare the package format with the NPM relationship described in the issue. Review how Dependabot would discover packages from git repositories, OpenUPM, or UnityNuGet, and define the supported update behavior and completion criteria before implementation.

Written by the indexing model from the issue text.