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.

首先跟踪 partition-field 模型及其序列化，然后继续检查 scan-planning 对 unknown transforms 的处理。将当前带单个参数的 source-id 行为与 V3 规范进行比较。完成的标准是：多参数 source-ids 能够正确加载和序列化，在 scan-planning 期间被视为未知，并且 write-time evaluation 仍不在范围内。

由索引模型根据 Issue 内容生成。