Make all methods async def again; add completion() for meta-reference (#270)

PR #201 had made several changes while trying to fix issues with getting the stream=False branches of inference and agents API working. As part of this, it made a change which was slightly gratuitous. Namely, making chat_completion() and brethren "def" instead of "async def".

The rationale was that this allowed the user (within llama-stack) of this to use it as:

```
async for chunk in api.chat_completion(params)
```

However, it causes unnecessary confusion for several folks. Given that clients (e.g., llama-stack-apps) anyway use the SDK methods (which are completely isolated) this choice was not ideal. Let's revert back so the call now looks like:

```
async for chunk in await api.chat_completion(params)
```

Bonus: Added a completion() implementation for the meta-reference provider. Technically should have been another PR :)

llama_stack/providers/impls/meta_reference/agents/agents.py

@@ -105,7 +105,7 @@ class MetaReferenceAgentsImpl(Agents):
             session_id=session_id,
         )
 
-    def create_agent_turn(
+    async def create_agent_turn(
         self,
         agent_id: str,
         session_id: str,