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

@@ -21,4 +21,6 @@ common --enable_runfiles
 common --registry=file:///%workspace%/misc/bazel/registry
 common --registry=https://bcr.bazel.build
 
+common --@rules_dotnet//dotnet/settings:strict_deps=false
+
 try-import %workspace%/local.bazelrc