feat: wire Stainless preview SDK into integration tests (#4360)

# What does this PR do?

Enable stainless-builds workflow to test preview SDKs by calling
integration-tests workflow with python_url parameter. Add stainless
matrix config for faster CI runs on SDK changes.

  - Make integration-tests.yml reusable with workflow_call inputs
  - Thread python_url through test setup actions to install preview SDK
- Add matrix_key parameter to generate_ci_matrix.py for custom matrices
- Update stainless-builds.yml to call integration tests with preview URL

This allows us to test a client on the PR introducing the new changes
before merging. Contributors can even write new tests using the
generated client which should pass on the PR, indicating that they will
pass on main upon merge

## Test Plan

see triggered action using the workflows on this branch:
5810594042
which installs the stainless SDK from the given url.

---------

Signed-off-by: Charlie Doern <cdoern@redhat.com>

tests/integration/ci_matrix.json

@@ -6,6 +6,9 @@
     {"suite": "responses", "setup": "gpt"},
     {"suite": "base-vllm-subset", "setup": "vllm"}
   ],
+  "stainless": [
+    {"suite": "base", "setup": "ollama", "allowed_clients": ["library"]}
+  ],
   "schedules": {
     "1 0 * * 0": [
       {"suite": "base", "setup": "vllm"}