Implementation of .NET packages.config extractor and fixes

[Octaviusss]

Pull request for the .NET packages.config extractor.
Implemented:

Extractor implementation for package.config;
Unit test implementation for extractor:
Integration of the extractor in the main tool;
Fixes from the .NET PR #355.

[vpasdf]

QQ: Are those files always that small? In case they can get more complex, please add a bit more complex case. If not, this is fine

[vpasdf]

Usually each result type of plugins has their own metadata struct. This is used in some use cases of Scalibr to distinguish result types

So you either add a metadata proto for .net. Then you need to update proto.go and scanresult proto. This is used in some programs to differentiate different result types. It's also recommended to have all necessary information in metadata for matching.

Or you don't, then that change is not required.

[Octaviusss]

We chose not to introduce a new metadata structure for the .NET extractor because the existing implementation (see .NET packages lock json) already captures the necessary fields required for accurate extraction and matching. The current extractor effectively handles the .NET without the need for additional fields at this time.
So we think that this change is not required.