Add OpenAPI workflow to confirm it's working

Signed-off-by: Joas Schilling <coding@schilljs.com>

.github/workflows/openapi.yml

@@ -0,0 +1,34 @@
+name: OpenAPI
+
+on: pull_request
+
+permissions:
+  contents: read
+
+concurrency:
+  group: openapi-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  openapi:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744 # v3.6.0
+        with:
+          submodules: true
+
+      - name: Set up php
+        uses: shivammathur/setup-php@4bd44f22a98a19e0950cbad5f31095157cc9621b # v2
+        with:
+          php-version: '8.1'
+          # https://docs.nextcloud.com/server/stable/admin_manual/installation/source_installation.html#prerequisites-for-manual-installation
+          extensions: bz2, ctype, curl, dom, fileinfo, gd, iconv, intl, json, libxml, mbstring, openssl, pcntl, posix, session, simplexml, xmlreader, xmlwriter, zip, zlib
+          coverage: none
+          ini-file: development
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build using Makefile
+        run: make openapi-spec