externals: Add catch2 v3.2.1

Merge commit '6879e5bb1c598a9a517d7bdec8ba1a0bace3ef10' as 'externals/catch'

externals/catch/.github/workflows/linux-bazel-builds.yml

@@ -0,0 +1,24 @@
+name: Bazel build
+
+on: [push, pull_request]
+
+jobs:
+  build_and_test_ubuntu:
+    name: Linux Ubuntu 22.04 Bazel build <GCC 11.2.0>
+    runs-on: ubuntu-22.04
+    strategy:
+      matrix:
+        compilation_mode: [fastbuild, dbg, opt]
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Mount bazel cache
+      uses: actions/cache@v3
+      with:
+        path: "/home/runner/.cache/bazel"
+        key: bazel-ubuntu22-gcc11
+
+    - name: Build Catch2
+      run: |
+        bazelisk build --compilation_mode=${{matrix.compilation_mode}} //...