Async API for RestCatalog

### Feature Request / Improvement

RestCatalog currently uses synchronous HTTP calls, while most catalog operations are network io-bound. Native async support would help async consumers and high-concurrency use cases in general.

We could introduce a separate AsyncRestCatalog, preserving the existing synchronous API.

asyncio.to_thread()/running in a dedicated executor is a possible workaround but doesn't give the same benefits as native support as we'd still have the limitations of the sync client (one worker thread per in-flight request being the most annoying in this case).

We can start small without necessarily the full ops coverage, wdyt?

Beginnen Sie bei der bestehenden RestCatalog-Implementierung und erfassen Sie, welche Katalogoperationen derzeit synchrone HTTP-Aufrufe verwenden. Definieren Sie den anfänglichen Umfang von AsyncRestCatalog und behalten Sie dabei die synchrone API bei; das Issue lässt den genauen Umfang der Operationen und den Validierungsansatz offen.

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