create a reusable workflow

.github/workflows/reusable_library_generation.yaml

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+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# GitHub action job to test core java library features on
+# downstream client libraries before they are released.
+name: Hermetic library generation upon generation config change through pull requests
+on:
+  workflow_call:
+  inputs:
+    repo_full_name:
+      required: true
+      type: string
+    github_repository:
+      required: true
+      type: string
+    base_ref:
+      required: true
+      type: string
+    head_ref:
+      required: true
+      type: string
+  secrets:
+    token:
+      required: true
+env:
+  REPO_FULL_NAME: ${{ inputs.repo_full_name }}
+  GITHUB_REPOSITORY: ${{ inputs.github_repository }}
+jobs:
+  library_generation_impl:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Prerequisite
+      shell: bash
+      run: |
+        if [[ "${GITHUB_REPOSITORY}" != "${REPO_FULL_NAME}" ]]; then
+          echo "This PR comes from a fork. Skip library generation."
+          exit 0
+        fi
+    - uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
+        token: ${{ secrets.token }}
+    - uses: ./library_generation
+    - name: Generate changed libraries
+      shell: bash
+      run: |
+        set -x
+        [ -z "$(git config user.email)" ] && git config --global user.email "cloud-java-bot@google.com"
+        [ -z "$(git config user.name)" ] && git config --global user.name "cloud-java-bot"
+        bash hermetic_library_generation.sh \
+          --target_branch "${BASE_REF}" \
+          --current_branch "${HEAD_REF}"
+      env:
+        BASE_REF: ${{ inputs.base_ref }}
+        HEAD_REF: ${{ inputs.head_ref }}
+        GH_TOKEN: ${{ secrets.token }}