Public headers require C++23, forcing the same on every consumer

`CMakeLists.txt:35` sets `CMAKE_CXX_STANDARD 23`, which is a fine choice for the
library's own sources. The requirement is not confined to them, though.
`src/iceberg/result.h` includes `` and ``, and defines:

```cpp
template ::type>
using Result = std::expected;

using Status = Result;
```

`Result` is the return type of most public entry points —
`Catalog::ListNamespaces`, `ListTables`, `LoadTable`, `CreateTable`,
`StageCreateTable` and so on. Every consumer translation unit that calls them
must therefore compile as C++23 as well. `std::expected` needs libstdc++ 12 or
libc++ 16 and `` needs libstdc++ 13, so a consumer on an older but still
widely deployed toolchain cannot include the headers at all.

This matters for the engines the library is meant to be embedded in: Velox
builds as C++20, Arrow as C++17, DuckDB as C++11. Integrating leaves two
options — move the whole engine to C++23, or add an isolation layer whose only
purpose is keeping iceberg-cpp headers out of the rest of the build. We are
looking at iceberg-cpp for an Iceberg connector in
[Axiom](https://github.com/facebookincubator/axiom), which builds on Velox at
C++20, and would rather do neither.

Would you consider making the public error type portable while keeping the API
shape unchanged?

```cpp
#if defined(__cpp_lib_expected)
template ::type>
using Result = std::expected;
#else
// vendored fallback with the same interface
#endif
```

`arrow::Result`, `absl::StatusOr` and `tl::expected` all exist for this reason.
The library's own sources could keep building as C++23; only the public headers
would need to hold a lower baseline.

Happy to send a patch if the direction is agreeable.

cc @PingLiuPing, who is proposing the Iceberg connector for Axiom.

CMakeLists.txt:35 と公開ヘッダー src/iceberg/result.h から始め、Result を公開する、一覧にある Catalog のエントリポイントをたどってください。必要な C++23 ライブラリ機能と、issue で指定されている古い toolchain を比較してください。ライブラリ自体のソースは引き続き C++23 としてビルドでき、API の形状は変わらないまま、利用者が C++23 を採用せずに公開 API をインクルードして使用できれば完了です。

索引モデルが issue の本文から書いたものです。