Kill "remote" providers and fix testing with a remote stack properly (#435)

# What does this PR do?

This PR kills the notion of "pure passthrough" remote providers. You
cannot specify a single provider you must specify a whole distribution
(stack) as remote.

This PR also significantly fixes / upgrades testing infrastructure so
you can now test against a remotely hosted stack server by just doing

```bash
pytest -s -v -m remote  test_agents.py \
  --inference-model=Llama3.1-8B-Instruct --safety-shield=Llama-Guard-3-1B \
  --env REMOTE_STACK_URL=http://localhost:5001
```

Also fixed `test_agents_persistence.py` (which was broken) and killed
some deprecated testing functions.

## Test Plan

All the tests.

llama_stack/providers/registry/memory.py

@@ -53,6 +53,7 @@ def available_providers() -> List[ProviderSpec]:
                 adapter_type="chromadb",
                 pip_packages=EMBEDDING_DEPS + ["chromadb-client"],
                 module="llama_stack.providers.remote.memory.chroma",
+                config_class="llama_stack.distribution.datatypes.RemoteProviderConfig",
             ),
         ),
         remote_provider_spec(