Use uv pip install instead of pip install (#921)

## What does this PR do? 

See issue: #747 -- `uv` is just plain better. This PR does the bare
minimum of replacing `pip install` by `uv pip install` and ensuring `uv`
exists in the environment.

## Test Plan 

First: create new conda, `uv pip install -e .` on `llama-stack` -- all
is good.
Next: run `llama stack build --template together` followed by `llama
stack run together` -- all good
Next: run `llama stack build --template together --image-name yoyo`
followed by `llama stack run together --image-name yoyo` -- all good
Next: fresh conda and `uv pip install -e .` and `llama stack build
--template together --image-type venv` -- all good.

Docker: `llama stack build --template together --image-type container`
works!

llama_stack/distribution/client.py

@@ -68,9 +68,9 @@ def create_api_client_class(protocol) -> Type:
                 return_type = None
             else:
                 return_type = extract_non_async_iterator_type(sig.return_annotation)
-                assert (
-                    return_type
-                ), f"Could not extract return type for {sig.return_annotation}"
+                assert return_type, (
+                    f"Could not extract return type for {sig.return_annotation}"
+                )
 
             async with httpx.AsyncClient() as client:
                 params = self.httpx_request_params(method_name, *args, **kwargs)
@@ -87,9 +87,9 @@ def create_api_client_class(protocol) -> Type:
             webmethod, sig = self.routes[method_name]
 
             return_type = extract_async_iterator_type(sig.return_annotation)
-            assert (
-                return_type
-            ), f"Could not extract return type for {sig.return_annotation}"
+            assert return_type, (
+                f"Could not extract return type for {sig.return_annotation}"
+            )
 
             async with httpx.AsyncClient() as client:
                 params = self.httpx_request_params(method_name, *args, **kwargs)