Python: provide get_chat_message_content (singular) method

[madsbolaris]

Instead of having to always get the first message like so...

result = (await chat_completion.get_chat_message_contents(
    chat_history=history,
    settings=execution_settings,
    kernel=kernel,
    arguments=KernelArguments(),
))[0]

I should be able to just do the following (like in .NET)

result = await chat_completion.get_chat_message_content(
    chat_history=history,
    settings=execution_settings,
    kernel=kernel,
    arguments=KernelArguments(),
)