Add Python version of crossmint-headless-checkout plugin

[devin-ai-integration]

Created a Python version of the crossmint-headless-checkout plugin with examples matching the TypeScript implementation.

Link to Devin run: https://app.devin.ai/sessions/c679e71dc4a24110a1dec045413df0d0
Requested by: Agus Armellini Fischer (agustin@paella.dev)

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Latest commit: 323611c

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[devin-ai-integration]

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