Check downstream backwards compatibility (#600)

## Changes
Comments on PR whenever downstream fails to compile or fails unit tests.
See implementation for more details:
- databrickslabs/sandbox#141

Backwards incompatible changes would result in comments similar to:
<img width="832" alt="image"
src="https://github.com/databricks/databricks-sdk-py/assets/259697/b1554491-6a80-417b-a2be-1350fa09b424">

Current downstream checking graph:

```mermaid
graph TD

pysdk[Python SDK: ***this PR***] --> ucx
pysdk --> blueprint

blueprint --> lsql
blueprint --> ucx
blueprint --> remorph

lsql --> ucx
lsql --> remorph
```


## Tests

Manual

---------

Signed-off-by: Serge Smertin <[email protected]>

Author: nfx

.github/workflows/downstreams.yml

@@ -0,0 +1,54 @@
+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 }}