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?

既存の RestCatalog 実装から始め、現在どのカタログ操作が同期 HTTP 呼び出しを使用しているかを整理します。同期 API を維持しながら AsyncRestCatalog の初期スコープを定義します。この issue では、操作の正確な対象範囲と検証アプローチは未確定のままです。

索引モデルが issue の本文から書いたものです。