[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/apis/agents/event_logger.py

@@ -137,7 +137,7 @@ class EventLogger:
                                     event,
                                     LogEvent(
                                         role=None,
-                                        content=delta.content,
+                                        content=delta.tool_call,
                                         end="",
                                         color="cyan",
                                     ),