[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=<...>)`

tests/client-sdk/safety/test_safety.py

@@ -141,7 +141,7 @@ def test_safety_with_image(llama_stack_client, model_providers):
                 },
                 {
                     "type": "image",
-                    "url": {"uri": data_url_from_image(file_path)},
+                    "image": {"url": {"uri": data_url_from_image(file_path)}},
                 },
             ],
         }