Do not generate, instantiate, nor carry Arrow extensions around

[teh-cmc]

• Do not generate, instantiate, nor carry Arrow extensions around: we do not need them, ever. We rely on chunk metadata for everything meta-esque.
• Do not let datatypes pretend that they have semantics. They do not need names.
• Clean everything accordingly.

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• DNM: requires #8151

Checklist

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• [x] If applicable, add a new check to the release checklist!

• [x] If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

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[teh-cmc]

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Started a full build: https://github.com/rerun-io/rerun/actions/runs/11842783240