Add support for C# compiled by .NET SDK

I tried adding support for C# compiled by .NET SDK 7.0. I have a very preliminary version running in the Czech national olympiad, but before I submit a pull request, I would like to discuss some problems I stumbled upon and agree on how they should be solved properly.

(1) Permission problems. The dotnet compiler creates a whole forest of intermediate directories, often with restrictive permissions. This hits the issue #1242. I worked around this by creating a temporary directory within /box and symlinking it as obj in the current directory.

(2) User lookup problem. The compiler expects that the current UID has an entry in /etc/passwd. With a typical installation of Isolate (as installed by CMS installation scripts), the sandbox users don't have their entries. As CMS bind-mounts /etc in compilation sandboxes, this can be worked around by adding all sandbox users to /etc/passwd of the host system. I wonder if this has a nicer solution...

(3) To use dotnet build, one must create a *.cproj project file first. I figured out how to do it, but not how to integrate it nicely with the CMS language modules: they allow generating compilation commands, but I would rather need to call a Python function which would generate the file and put it in the sandbox directory. I worked around it by calling a helper script, but then we have to install the helper script to a location which is available inside the sandbox (e.g., /usr/local/bin), which is different from where we install the CMS itself (in my case, it's a virtual environment).

(4) I need to modify some parameters of the sandbox: increase the maximum number of open files, pass some extra environment variables, possibly bind-mount more directories. I did it by adding these options to all languages, but it would be really nice to have them per-language. I handled environment variables in the helper script for the time being.

I would appreciate any ideas on how to solve these problems in an upstreamable way.

Read the CMS language modules and sandbox configuration entry points first, along with issue #1242. The work is not yet a defined implementation: it needs an agreed upstreamable approach for permissions, user lookup, project-file generation, and per-language sandbox settings.