Add helper to simplify selecting ChatModel

[jackmpcollins]

To avoid long import paths and make it easier to change model using just a string, add a model function that returns the appropriate ChatModel instance based on the name provided. This should also accept all other model args (temperature, max_tokens, etc.). For models that do not support the provided args we can raise an error, warn, or ignore, and this should be configurable by a parameter.

This function should be used for creation of the default ChatModel from env vars, to keep behavior consistent. Including giving a clear error message if the required dependency is not installed. Related issue: #415

There should be a way to register new ChatModel classes for users that create their own.

Usage

from magentic import model, prompt


@prompt("Say hello")
def say_hello() -> str: ...


@prompt(
    "Say hello",
    model=model("anthropic:claude-3-5-sonnet-latest"),
    # And even just
    model="anthropic:claude-3-5-sonnet-latest",
)
def say_hello_anthropic() -> str: ...


say_hello()  # Uses env vars or default settings

with model("openai:gpt-4o-mini", temperature=1):
    say_hello()  # Uses openai with gpt-4o-mini and temperature=1 due to context manager
    say_hello_anthropic()  # Uses Anthropic claude-3-5-sonnet-latest because explicitly configured