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/update-deps.sh

@@ -0,0 +1,16 @@
+#! /usr/bin/env bash
+
+SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+MODE=${1:-install}
+if [ "$MODE" != "install" ] && [ "$MODE" != "update" ]; then
+    echo "Invalid mode: $MODE. Valid modes are 'install' and 'update'"
+    exit 1
+fi
+
+(
+    cd "$SCRIPT_DIR" || exit 1
+    DEP_DIR="$(pwd)"
+    (dotnet tool restore && dotnet paket $MODE) || exit 1
+    cd ../..
+    tools/bazel run @rules_dotnet//tools/paket2bazel -- --dependencies-file "$DEP_DIR"/paket.dependencies --output-folder "$DEP_DIR"
+)