Allow using an "inline" version of Chroma using PersistentClient (#567)

The same code is used (inside providers/remote/memory/chroma/chroma.py)
but it is driven by separate configurations and changes which Chroma
client to use. Note that the dependencies are separate
(`chromadb-client` vs `chromadb` -- the latter is a _much_ heavier
package.)

```
pytest -s -v -m chroma memory/test_memory.py --env CHROMA_DB_PATH=/tmp/chroma_test
pytest -s -v -m chroma memory/test_memory.py --env CHROMA_URL=http://localhost:6001
```

llama_stack/providers/remote/memory/chroma/config.py

@@ -0,0 +1,17 @@
+# 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.
+
+from typing import Any, Dict
+
+from pydantic import BaseModel
+
+
+class ChromaRemoteImplConfig(BaseModel):
+    url: str
+
+    @classmethod
+    def sample_config(cls) -> Dict[str, Any]:
+        return {"url": "{env.CHROMADB_URL}"}