Allow Messages as input to prompt_chain

[jackmpcollins]

Also update docs for this.

Closes #389
Closes #391
Closes #392

Thanks to @alexchandel for the suggestion!

Example

from magentic import prompt_chain, UserMessage

def get_current_weather(location, unit="fahrenheit"):
    """Get the current weather in a given location"""
    return {"temperature": "72", "forecast": ["sunny", "windy"]}

@prompt_chain(
    template=[UserMessage("What's the weather like in {city}?")],
    functions=[get_current_weather],
)
def describe_weather(city: str) -> str: ...

describe_weather("Boston")
'The weather in Boston is currently 72°F with sunny and windy conditions.'