refactor(test): introduce --stack-config and simplify options (#1404)

You now run the integration tests with these options:

```bash
Custom options:
  --stack-config=STACK_CONFIG
                        a 'pointer' to the stack. this can be either be:
                        (a) a template name like `fireworks`, or
                        (b) a path to a run.yaml file, or
                        (c) an adhoc config spec, e.g.
                        `inference=fireworks,safety=llama-guard,agents=meta-
                        reference`
  --env=ENV             Set environment variables, e.g. --env KEY=value
  --text-model=TEXT_MODEL
                        comma-separated list of text models. Fixture name:
                        text_model_id
  --vision-model=VISION_MODEL
                        comma-separated list of vision models. Fixture name:
                        vision_model_id
  --embedding-model=EMBEDDING_MODEL
                        comma-separated list of embedding models. Fixture name:
                        embedding_model_id
  --safety-shield=SAFETY_SHIELD
                        comma-separated list of safety shields. Fixture name:
                        shield_id
  --judge-model=JUDGE_MODEL
                        comma-separated list of judge models. Fixture name:
                        judge_model_id
  --embedding-dimension=EMBEDDING_DIMENSION
                        Output dimensionality of the embedding model to use for
                        testing. Default: 384
  --record-responses    Record new API responses instead of using cached ones.
  --report=REPORT       Path where the test report should be written, e.g.
                        --report=/path/to/report.md

```

Importantly, if you don't specify any of the models (text-model,
vision-model, etc.) the relevant tests will get **skipped!**

This will make running tests somewhat more annoying since all options
will need to be specified. We will make this easier by adding some easy
wrapper yaml configs.

## Test Plan

Example:

```bash
ashwin@ashwin-mbp ~/local/llama-stack/tests/integration (unify_tests) $ 
LLAMA_STACK_CONFIG=fireworks pytest -s -v inference/test_text_inference.py \
   --text-model meta-llama/Llama-3.2-3B-Instruct 
```

tests/__init__.py

@@ -0,0 +1,5 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.