Simplify creation of few-shot chatprompts with function calls

[jackmpcollins]

The same FunctionCall must be used in AssistantMessage and FunctionResultMessage for them to have the same tool_call_id. The current syntax for doing this could be cleaned up to avoid creating the awkward FunctionCall instance(s) above the chatprompt-function.

Example from the docs https://magentic.dev/chat-prompting/#functionresultmessage

def plus(a: int, b: int) -> int:
    return a + b


plus_1_2 = FunctionCall(plus, 1, 2)


@chatprompt(
    UserMessage("Use the plus function to add 1 and 2."),
    AssistantMessage(plus_1_2),
    FunctionResultMessage(3, plus_1_2),
    UserMessage("Now add 4 to the result."),
    functions=[plus],
)
def do_math() -> FunctionCall[int]: ...

ideally could more like (moving the creation of the FunctionCall inside)

def plus(a: int, b: int) -> int:
    return a + b


@chatprompt(
    UserMessage("Use the plus function to add 1 and 2."),
    *assistant_function_call_and_result(3, plus, 1, 2),
    UserMessage("Now add 4 to the result."),
    functions=[plus],
)
def do_math() -> FunctionCall[int]: ...

Also need to consider ParallelFunctionCall, and possibly future tools too.