Added downstreams action

[nfx]

Comments on PR whenever backwards compatibility is broken:

And fails the build:

Usage:

name: downstreams

on:
  pull_request:
    types: [opened, synchronize]
  merge_group:
    types: [checks_requested]
  push:
    # Always run on push to main. The build cache can only be reused
    # if it was saved by a run from the repository's default branch.
    # The run result will be identical to that from the merge queue
    # because the commit is identical, yet we need to perform it to
    # seed the build cache.
    branches:
      - main

permissions:
  id-token: write
  contents: read
  pull-requests: write

jobs:
  compatibility:
    strategy:
      fail-fast: false
      matrix:
        downstream:
          - name: ucx
            org: databrickslabs
          - name: blueprint
            org: databrickslabs
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
        uses: actions/checkout@v3
        with:
          fetch-depth: 0

      - name: Install Python
        uses: actions/setup-python@v4
        with:
          python-version: '3.10'

      - name: Install toolchain
        run: |
          pip install hatch==1.9.4
      - name: Acceptance
        uses: databrickslabs/sandbox/downstreams@downstreams/v0.0.1
        with:
          repo: ${{ matrix.downstream.name }}
          org: ${{ matrix.downstream.org }}
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}