feat: github action 자바 코드 형식

backend/core/.github/workflows/google-java-format.yml

@@ -0,0 +1,16 @@
+name: Format
+
+on: [ push, pull_request ]
+
+jobs:
+
+  formatting:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: axel-op/googlejavaformat-action@v3
+        with:
+          args: "--replace"
+          skip-commit: true
+      - name: Print diffs
+        run: git --no-pager diff --exit-code