Add a swift-format check to run on PRs.

Based off the swift-nio ones.

.github/workflows/swift-format.yml

@@ -0,0 +1,30 @@
+name: swift-format Check
+
+on:
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  format-check:
+    name: swift-format Check
+    runs-on: ubuntu-latest
+    container:
+      # Use use the latest Swift release and that's the version of swift-format
+      # people should use.
+      image: swift:latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+    - name: Mark the workspace as safe
+      # https://github.com/actions/checkout/issues/766
+      run: git config --global --add safe.directory ${GITHUB_WORKSPACE}
+    - name: Run format check
+      run:  |
+        set -eu
+        git ls-files -z '*.swift' | xargs -0 swift format format --parallel --in-place
+        GIT_PAGER='' git diff --exit-code '*.swift'
+    # Disabled as it produces multiple warnings at the moment.
+    # - name: Run format lint check
+    #   run:  |
+    #     set -eu
+    #     git ls-files -z '*.swift' | xargs -0 swift format lint --strict --parallel