Add VCR setup helpers for testing

[jackmpcollins]

For testing, magentic configures VCR to ignore LLM provider API keys, and also to match on the whole request body. This seems like broadly useful configuration so could be exposed as helper functions, maybe scoped down to just affect requests to openai/anthropic/etc urls.

Under magentic.testing (like numpy and pandas have).

Add docs for using VCR for testing / recommended testing setup.