C#: Add Bazel-based build system.

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.

.bazelrc.internal

@@ -2,3 +2,5 @@
 
 common --registry=file:///%workspace%/ql/misc/bazel/registry
 common --registry=https://bcr.bazel.build
+
+common --@rules_dotnet//dotnet/settings:strict_deps=false