ci: run downstream client library dependency tests

.github/workflows/downstream.yaml

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+on:
+  push:
+    branches:
+      - master
+  pull_request:
+name: downstream
+jobs:
+  dependencies:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        java: [8, 11]
+        repo:
+          - java-bigquery
+          - java-bigqueryconnection
+          - java-spanner
+          - java-storage
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-java@v1
+        with:
+          java-version: ${{matrix.java}}
+      - run: java -version
+      - run: sudo apt-get install libxml2-utils
+      - run: .kokoro/client-library-check.sh ${{matrix.repo}}

.kokoro/client-library-check.sh

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+#!/bin/bash
+# Copyright 2020 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.
+
+# Presubmit to ensure the dependencies of the Google Libraries BOM, with the modification of change
+# in the PR, pick up the highest versions among transitive dependencies.
+# https://maven.apache.org/enforcer/enforcer-rules/requireUpperBoundDeps.html
+
+set -eo pipefail
+# Display commands being run.
+set -x
+
+if [[ $# -lt 1 ]];
+then
+  echo "Usage: $0 <repo-name>"
+  exit 1
+fi
+REPO=$1
+
+## Get the directory of the build script
+scriptDir=$(realpath $(dirname "${BASH_SOURCE[0]}"))
+## cd to the parent directory, i.e. the root of the git repo
+cd ${scriptDir}/..
+
+# Make artifacts available for 'mvn validate' at the bottom
+mvn install -DskipTests=true -Dmaven.javadoc.skip=true -Dgcloud.download.skip=true -B -V -q
+
+# Read the current version of this BOM in the POM. Example version: '0.116.1-alpha-SNAPSHOT'
+VERSION_POM=pom.xml
+# Namespace (xmlns) prevents xmllint from specifying tag names in XPath
+VERSION=`sed -e 's/xmlns=".*"//' ${VERSION_POM} | xmllint --xpath '/project/version/text()' -`
+
+if [ -z "${VERSION}" ]; then
+  echo "Version is not found in ${VERSION_POM}"
+  exit 1
+fi
+echo "Version: ${VERSION}"
+
+# Check this BOM against a few java client libraries
+# java-bigquery
+git clone "https://github.com/googleapis/${REPO}.git" --depth=1
+pushd ${REPO}
+
+# replace version
+xmllint --shell <(cat pom.xml) << EOF
+setns x=http://maven.apache.org/POM/4.0.0
+cd .//x:artifactId[text()="google-cloud-shared-dependencies"]
+cd ../x:version
+set ${VERSION}
+save pom.xml
+EOF
+
+# run dependencies script
+.kokoro/dependencies.sh

.kokoro/presubmit/client-library-check.cfg

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+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Configure the docker image for kokoro-trampoline.
+env_vars: {
+    key: "TRAMPOLINE_IMAGE"
+    value: "gcr.io/cloud-devrel-kokoro-resources/java8"
+}
+
+env_vars: {
+    key: "TRAMPOLINE_BUILD_FILE"
+    value: "github/java-shared-dependencies/.kokoro/client-library-check.sh"
+}