Add support for OpenAI structured outputs

[jackmpcollins]

• Extend pydantic's ConfigDict to add openai_strict: bool
• Enable pydantic config to be set on functions
• For all types, pass along the pydantic config (if present) to the pydantic model used for serialization in FunctionSchemas
• Try read pydantic config when converting type to JSON schema, and use openai strict converter if openai_strict is True
• Add to docs

Example

from magentic import prompt, ConfigDict
from pydantic import BaseModel


class Superhero(BaseModel):
    model_config = ConfigDict(openai_strict=True)

    name: str
    age: int
    power: str
    enemies: list[str]


@prompt("Create a Superhero named {name}.")
def create_superhero(name: str) -> Superhero: ...


create_superhero("Garden Man")

See docs for more: http://localhost:8000/structured-outputs/#configdict

Related to #295