[test automation] support run tests on config file (#730)

# Context
For test automation, the end goal is to run a single pytest command from
root test directory (llama_stack/providers/tests/.) such that we execute
push-blocking tests

The work plan: 
1) trigger pytest from llama_stack/providers/tests/.
2) use config file to determine what tests and parametrization we want
to run

# What does this PR do?
1) consolidates the "inference-models" / "embedding-model" /
"judge-model" ... options in root conftest.py. Without this change, we
will hit into error when trying to run `pytest
/Users/sxyi/llama-stack/llama_stack/providers/tests/.` because of
duplicated `addoptions` definitions across child conftest files.

2) Add a `config` option to specify test config in YAML. (see
[`ci_test_config.yaml`](https://gist.github.com/sixianyi0721/5b37fbce4069139445c2f06f6e42f87e)
for example config file)

For provider_fixtures, we allow users to use either a default fixture
combination or define their own {api:provider} combinations.

```

memory:
....
  fixtures:
    provider_fixtures:
    - default_fixture_param_id: ollama // use default fixture combination with param_id="ollama" in [providers/tests/memory/conftest.py](https://fburl.com/mtjzwsmk)
    - inference: sentence_transformers
      memory: faiss
    - default_fixture_param_id: chroma

```
3) generate tests according to the config. Logic lives in two places: 
a) in `{api}/conftest.py::pytest_generate_tests`, we read from config to
do parametrization.
b) after test collection, in `pytest_collection_modifyitems`, we filter
the tests to include only functions listed in config.

## Test Plan
1) `pytest /Users/sxyi/llama-stack/llama_stack/providers/tests/.
--collect-only --config=ci_test_config.yaml`

Using `--collect-only` tag to print the pytests listed in the config
file (`ci_test_config.yaml`).

output:
[gist](https://gist.github.com/sixianyi0721/05145e60d4d085c17cfb304beeb1e60e)


2) sanity check on `--inference-model` option

```
pytest -v -s -k "ollama" --inference-model="meta-llama/Llama-3.1-8B-Instruct" ./llama_stack/providers/tests/inference/test_text_inference.py
```


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [x] Ran pre-commit to handle lint / formatting issues.
- [x] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/providers/tests/scoring/conftest.py

@@ -55,21 +55,6 @@ def pytest_configure(config):
         )
 
 
-def pytest_addoption(parser):
-    parser.addoption(
-        "--inference-model",
-        action="store",
-        default="meta-llama/Llama-3.2-3B-Instruct",
-        help="Specify the inference model to use for testing",
-    )
-    parser.addoption(
-        "--judge-model",
-        action="store",
-        default="meta-llama/Llama-3.1-8B-Instruct",
-        help="Specify the judge model to use for testing",
-    )
-
-
 def pytest_generate_tests(metafunc):
     judge_model = metafunc.config.getoption("--judge-model")
     if "judge_model" in metafunc.fixturenames: