[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/nvidia/openai_utils.py

@@ -505,7 +505,9 @@ async def convert_openai_chat_completion_stream(
                 event=ChatCompletionResponseEvent(
                     event_type=next(event_type),
                     delta=ToolCallDelta(
-                        content=_convert_openai_tool_calls(choice.delta.tool_calls)[0],
+                        tool_call=_convert_openai_tool_calls(choice.delta.tool_calls)[
+                            0
+                        ],
                         parse_status=ToolCallParseStatus.succeeded,
                     ),
                     logprobs=_convert_openai_logprobs(choice.logprobs),