Support multi-argument transforms

### Feature Request / Improvement

Part of #1818.

The V3 spec generalizes partition and sort fields to multiple source columns via `source-ids`. PyIceberg currently raises when `source-ids` has more than one element, so loading such tables fails — but the spec requires v3 readers to read tables with unknown transforms, ignoring them (null partition values, always-true projection in scan planning).

Scope: model `source-ids`, serialize per spec (`source-id` for single-argument transforms, `source-ids` otherwise), and treat multi-argument transforms as unknown in scan planning. Evaluating multi-argument transforms on write is out of scope until the spec defines concrete ones.

首先追蹤 partition-field 模型及其序列化，接著檢查 scan-planning 對 unknown transforms 的處理。將目前帶有單一參數的 source-id 行為與 V3 規範進行比較。完成的標準是：多參數 source-ids 能正確載入和序列化，在 scan-planning 期間被視為未知，且 write-time evaluation 仍不在範圍內。

由索引模型根據 Issue 內容生成。