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/apis/inference/client.py

@@ -42,10 +42,10 @@ class InferenceClient(Inference):
     async def shutdown(self) -> None:
         pass
 
-    def completion(self, request: CompletionRequest) -> AsyncGenerator:
+    async def completion(self, request: CompletionRequest) -> AsyncGenerator:
         raise NotImplementedError()
 
-    def chat_completion(
+    async def chat_completion(
         self,
         model: str,
         messages: List[Message],
@@ -139,7 +139,8 @@ async def run_main(
     else:
         logprobs_config = None
 
-    iterator = client.chat_completion(
+    assert stream, "Non streaming not supported here"
+    iterator = await client.chat_completion(
         model=model,
         messages=[message],
         stream=stream,