chore: add GCF buildpack integration test Workflow

See [functions-framework-conformance builidpack integration workflow PR](GoogleCloudPlatform/functions-framework-conformance#99) for more information.

Also, move the script for running conformance tests locally to the
conformance test directory.

.github/workflows/buildpack-integration-test.yml

@@ -0,0 +1,32 @@
+# Validates Functions Framework with GCF buildpacks.
+name: Buildpack Integration Test
+on:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+jobs:
+  php74-buildpack-test:
+    uses: GoogleCloudPlatform/functions-framework-conformance/.github/workflows/[email protected]
+    with:
+      http-builder-source: 'tests/conformance'
+      http-builder-target: 'declarativeHttpFunc'
+      cloudevent-builder-source: 'tests/conformance'
+      cloudevent-builder-target: 'declarativeCloudEventFunc'
+      prerun: 'tests/conformance/prerun.sh caff4321e38ac14cb158f3e2f01786609518ecd7'
+      output-file: 'vendor/bin/function_output.json'
+      builder-runtime: 'php74'
+      # Latest uploaded tag from us.gcr.io/fn-img/buildpacks/php74/builder
+      builder-tag: 'php74_20220620_7_4_29_RC00'
+  php81-buildpack-test:
+    uses: GoogleCloudPlatform/functions-framework-conformance/.github/workflows/[email protected]
+    with:
+      http-builder-source: 'tests/conformance'
+      http-builder-target: 'declarativeHttpFunc'
+      cloudevent-builder-source: 'tests/conformance'
+      cloudevent-builder-target: 'declarativeCloudEventFunc'
+      prerun: 'tests/conformance/prerun.sh caff4321e38ac14cb158f3e2f01786609518ecd7'
+      output-file: 'vendor/bin/function_output.json'
+      builder-runtime: 'php81'
+      # Latest uploaded tag from us.gcr.io/fn-img/buildpacks/php81/builder
+      builder-tag: 'php81_20220620_8_1_6_RC00'