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/inference.py

@@ -88,7 +88,8 @@ class CompletionRequest(BaseModel):
 class CompletionResponse(BaseModel):
     """Completion response."""
 
-    completion_message: CompletionMessage
+    content: str
+    stop_reason: StopReason
     logprobs: Optional[List[TokenLogProbs]] = None
 
 
@@ -113,7 +114,7 @@ class BatchCompletionRequest(BaseModel):
 class BatchCompletionResponse(BaseModel):
     """Batch completion response."""
 
-    completion_message_batch: List[CompletionMessage]
+    batch: List[CompletionResponse]
 
 
 @json_schema_type
@@ -165,7 +166,7 @@ class BatchChatCompletionRequest(BaseModel):
 
 @json_schema_type
 class BatchChatCompletionResponse(BaseModel):
-    completion_message_batch: List[CompletionMessage]
+    batch: List[ChatCompletionResponse]
 
 
 @json_schema_type
@@ -181,10 +182,8 @@ class ModelStore(Protocol):
 class Inference(Protocol):
     model_store: ModelStore
 
-    # This method is not `async def` because it can result in either an
-    # `AsyncGenerator` or a `CompletionResponse` depending on the value of `stream`.
     @webmethod(route="/inference/completion")
-    def completion(
+    async def completion(
         self,
         model: str,
         content: InterleavedTextMedia,
@@ -196,7 +195,7 @@ class Inference(Protocol):
     # This method is not `async def` because it can result in either an
     # `AsyncGenerator` or a `ChatCompletionResponse` depending on the value of `stream`.
     @webmethod(route="/inference/chat_completion")
-    def chat_completion(
+    async def chat_completion(
         self,
         model: str,
         messages: List[Message],