Add optional entity_id annotation

[padix-key]

As discussed in #732 (comment), this PR adds entity_id as optional AtomArray annotation that mirrors the atom_site.label_entity_id.

[codspeed-hq]

Merging this PR will not alter performance

✅ 98 untouched benchmarks

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_{Comparing padix-key:entity-id-annotation (a8c23e2) with main (2315f2a)}