Allow functions to return fully formed Parts that can be added directly to the content of the response

[slilichenko]

Currently, a function can only return a dictionary (or an object which gets converted to the dictionary). It prevents functions from returning more complex Parts, like images references or binary file contents and forces complex orchestration or adding these parts via callbacks.

[GWeale]

tools can now return a types.Part with inline media, on its own or inside the list or dict you already return, and adk lifts it into the function response. ships in the next release.