Support .NET (Core)

[timbussmann]

With .NET Core being supported on Linux, it would be great to use the actual .NET runtime instead of Mono. The latest LTS .NET version (6.0) would bring massive performance benefits and many new useful language features. Writing code for Mono is a lot more effort compared to just using .NET nowadays, both on Windows and Linux.

One major issue with .NET is that it's very difficult to compile single code files. Compiling a C# file usually requires a project file, otherwise, the dotnet commands will fail, and bypassing this with direct use of the compiler is brittle. The project file could easily be hard-coded or generated on the fly though.

I'd be happy to assist with this but given my lack of experience with the server implementation and Python, I'm struggling to get started. I've already proposed this PR to add the .NET 6 SDK to the runtimes image.

[Riolku]

Rust uses some amount of a project file (it has a Cargo.toml hardcoded), so maybe we could do something like that for .NET as well.

[timbussmann]

Thanks for the hint with the Rust executor @Riolku , that helped to get me started. I'm struggling to grasp how get_versionable_commands is working, is there any documentation that you can point me to?

[Riolku]

Go read base_executor.py. Seems the command returns the canonical name and path of all parts relevant to a runtime's version.

[timbussmann]

I've started a first draft via #1064, help and feedback are very welcome.