ci: Add Downstream Protobuf-Java Source Compatibility Test

.github/workflows/downstream_protobuf_source_compatibility_check_nightly.yaml

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+on:
+  workflow_dispatch:
+    inputs:
+      protobuf_versions:
+        description: 'Comma separated list of Protobuf-Java versions (i.e. "3.25.5","4.28.3")'
+        # Default value for workflow_dispatch flow. If updating default value, update below as well.
+        default: '"3.25.5","4.28.3"'
+        required: true
+  schedule:
+    - cron: '0 1 * * *' # Nightly at 1am
+
+name: Downstream Source Compatibility Nightly
+jobs:
+  downstream-protobuf-source-test:
+    runs-on: ubuntu-22.04
+    strategy:
+      fail-fast: false
+      matrix:
+        repo:
+          - google-cloud-java
+          - java-bigtable
+          - java-bigquery
+          - java-bigquerystorage
+          - java-datastore
+          - java-firestore
+          - java-logging
+          - java-logging-logback
+          - java-pubsub
+          - java-pubsublite
+          - java-spanner-jdbc
+          - java-spanner
+          - java-storage
+          - java-storage-nio
+        # Specify the Protobuf versions here as well because the default values above are only supplied from
+        # the workflow_dispatch flow. Without this, the nightly workflow doesn't have a default input otherwise
+        # and would resolve to ''. When updating, update both places to keep default values consistent.
+        protobuf-version: ${{ fromJSON(format('[{0}]', inputs.protobuf_versions || '"3.25.5","4.28.3"')) }}
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-java@v3
+        with:
+          java-version: 17
+          distribution: temurin
+      - run: mvn -version
+      - name: Perform downstream compatibility testing
+        run: REPOS_UNDER_TEST="${{ matrix.repo }}" PROTOBUF_RUNTIME_VERSION="${{ matrix.protobuf-version}}" ./.kokoro/presubmit/downstream-protobuf-source-compatibility.sh

.kokoro/presubmit/downstream-protobuf-source-compatibility.sh

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+#!/bin/bash
+# Copyright 2023 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.
+
+set -eo pipefail
+
+# Comma-delimited list of repos to test with the local java-shared-dependencies
+if [ -z "${REPOS_UNDER_TEST}" ]; then
+  echo "REPOS_UNDER_TEST must be set to run downstream-protobuf-source-compatibility.sh"
+  exit 1
+fi
+
+# Version of Protobuf-Java runtime to compile with
+if [ -z "${PROTOBUF_RUNTIME_VERSION}" ]; then
+  echo "PROTOBUF_RUNTIME_VERSION must be set to run downstream-protobuf-source-compatibility.sh"
+  exit 1
+fi
+
+# Get the directory of the build script
+scriptDir=$(realpath "$(dirname "${BASH_SOURCE[0]}")")
+cd "${scriptDir}/../.." # cd to the root of this repo
+source "$scriptDir/common.sh"
+
+setup_maven_mirror
+
+echo "Testing with Protobuf-Java v${PROTOBUF_RUNTIME_VERSION}"
+
+for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
+  # Perform source-compatibility testing on main (latest changes)
+  git clone "https://github.com/googleapis/$repo.git" --depth=1
+  pushd "$repo"
+
+  # Compile the Handwritten Library with the Protobuf-Java version to test source compatibility
+  # Run unit tests to help check for any behavior differences (dependant on coverage)
+  mvn clean test -B -V -ntp \
+      -Dclirr.skip=true \
+      -Denforcer.skip=true \
+      -Dmaven.javadoc.skip=true \
+      -Dprotobuf.version=${PROTOBUF_RUNTIME_VERSION} \
+      -T 1C
+  popd
+done