feat: Add reusable Workflow for buildpack integration test

[anniefu]

This can be used by Functions Frameworks to test the framework against
the official GCF builders, ensuring the frameworks will work containerized.

It runs HTTP tests similar to the local conformance test client runs, except in buildpack mode which:

1. Builds the function into a container using pack and the appropriate GCF builder
2. Runs the container on docker
3. Sends standard conformance test HTTP requests to the running container and verifies the response

Optimizations:

• Conformance test client is cached by client version, so it doesn't have to be rebuilt/re-installed unless there's a new release
• Each conformance test client run is run in its own GitHub job/runner in parallel

Example .github/workflows/buildpack-integration-test.yml that would go in an FF repo:

name: Buildpack Integration Test
on:
  # manually triggered for example
  workflow_dispatch:
jobs:
  buildpack-integration-test:
    uses: GoogleCloudPlatform/functions-framework-conformance/.github/workflows/[email protected]
    with:
      http-builder-source: 'testdata/conformance/function'
      http-builder-target: 'declarativeHTTP'
      cloudevent-builder-source: 'testdata/conformance/function'
      cloudevent-builder-target: 'declarativeCloudEvent'
      builder-runtime: 'go116'
      builder-tag: 'go116_20220320_1_16_13_RC00'