Allow using pydantic plugin with models defined before calling logfire.configure

[alexmojaki]

So that e.g. this just works:

import os
import pydantic

os.environ['LOGFIRE_PYDANTIC_PLUGIN_RECORD'] = 'all'

class A(pydantic.BaseModel):
    a: str

import logfire
logfire.configure()

A(a='a')

Motivation: https://pydanticlogfire.slack.com/archives/C06EDRBSAH3/p1714076434651649?thread_ts=1713995400.473799&cid=C06EDRBSAH3

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