Native support for Pydantic models for function_framework.typed

[haizaar]

Following #208 (comment),
it will be great to have Pydantic models supported natively. E.g.

from pydantic import BaseModel

class HelloRequest(BaseModel):
    name: str

@functions_framework.typed
def hello(hr: HelloRequest):
    return f"Hello, {hr.name}"

Thank you!