Allow initializing Rsmime with in-memory certificate data

[tiwilliam]

Summary

• allow the Rsmime constructor to accept PEM strings in addition to file paths and validate conflicting inputs
• document the new initialization option and update type stubs
• extend the test suite to cover in-memory materials, validation errors, and ignore local virtual environments

Testing

• .venv/bin/pytest

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https://chatgpt.com/codex/tasks/task_e_68f34a2dfacc8331868f63ae89e1bc42