Add OPTIMADE adapter

[ml-evs]

Summary

This PR adds a converter between the pymatgen.core.Structure object and an OPTIMADE structure resource, migrated primarily out of https://github.com/Materials-Consortia/optimade-python-tools/blob/master/optimade/adapters/structures/pymatgen.py with the aim to remove it from our library.

Todos

• [ ] Finish off tests
• [ ] See if it can be used internally in the pymatgen.ext.optimade client

Checklist

• [x] Google format doc strings added. Check with ruff.
• [x] Type annotations included. Check with mypy.
• [ ] Tests added for new features/fixes.
• [x] If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)