feat: refactor rest signer in fileio

### Feature Request / Improvement

Context: https://github.com/apache/iceberg-python/pull/2846#discussion_r2645920570

Currently S3 signer is tightly coupled with FileIO. This is awkward since the S3 Signer requires interaction with remote service (i.e. REST Catalog) and reuse its auth manager.

We can refactor here and create the signer in REST Catalog and pass to FileIO by overriding its `_load_file_io` function.

Also see [_init_sigv4](https://github.com/apache/iceberg-python/blob/a99dcada431a06329959cb7f71bf961343256bb0/pyiceberg/catalog/rest/__init__.py#L409-L459) for inspiration

PR 2846 の議論と、`pyiceberg/catalog/rest/__init__.py` にある `_init_sigv4` の実装を読んでください。REST Catalog の `_load_file_io` の経路と、現在 S3 signer が FileIO とどのように結合されているかを追ってください。REST Catalog が自身の認証マネージャーを使って signer を作成し、override を通じて FileIO に渡せば完了です。

索引モデルが issue の本文から書いたものです。