[inference api] modify content types so they follow a more standard structure (#841)

Some small updates to the inference types to make them more standard

Specifically:
- image data is now located in a "image" subkey
- similarly tool call data is located in a "tool_call" subkey

The pattern followed is `dict(type="foo", foo=<...>)`

llama_stack/providers/remote/inference/groq/groq_utils.py

@@ -218,7 +218,7 @@ async def convert_chat_completion_response_stream(
                 event=ChatCompletionResponseEvent(
                     event_type=event_type,
                     delta=ToolCallDelta(
-                        content=tool_call,
+                        tool_call=tool_call,
                         parse_status=ToolCallParseStatus.succeeded,
                     ),
                 )