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.

csharp/tools/BUILD.bazel

@@ -0,0 +1,11 @@
+load("@semmle_code//:dist.bzl", "pack_zip")
+
+pack_zip(
+    name = "tools",
+    srcs = glob(["**/*"]),
+    excludes = [
+        "BUILD.bazel",
+    ],
+    prefix = "tools",
+    visibility = ["//csharp:__pkg__"],
+)