This commits a bazel-based build system for C#
using `rules_dotnet`. External dependencies are managed
via `paket`, and updates to the generated bazel files
are done via `./update-deps.sh`.
We're providing our own (minimal) test runner for `xunit`
tests.
When running `dotnet paket update` or `dotnet paket install`, `paket`
forcefully reformats the project files. This is unfortunate.
One option is to accept these changes, as they're not harmful.
They do mean that each project includes the Paket restore targets individually,
instead of doing so via `Directory.Build.targets`.
Another option would be to not merge this PR, and then I'll document that
the changes to the csproj files should be ignored when running `paket` instead.
I don't really mind either way.
This is a necessary preparation for moving the C# dependency management to `paket`,
which in turn is a necessary preparation for moving the C# build to bazel.
As we discovered in https://github.com/github/codeql/pull/16376,
`paket` tries to restore all projects recursively from the root folder.
If we support building C# code under both `ql/csharp` and `ql/cpp`, we need
to have a single lockfile under `ql`, as both codebases share the same set of dependencies
(and utilities from `ql/csharp/extractor`).
Then, `paket` will also try to restore things that look like "C# projects" in other languages'
folders, which is not what we want.
Therefore, we address this by moving all C# code into a common root directory, `ql/csharp`.
This needs an internal PR to adjust the buildsystem to look for the autobuilder in the new location.
