Avro schema is re-converted to an Iceberg schema on every manifest read during scan planning

### Feature Request / Improvement

`AvroFileHeader.get_schema()` (`pyiceberg/avro/file.py`) runs the full `avro_to_iceberg` conversion every time an
Avro file is opened.

**The inefficiency**

- Every manifest under a spec embeds an *identical* Avro schema string.
- So during scan planning, that same conversion is repeated **once per manifest**.
- The cost grows with the manifest count, even though only a couple of distinct schemas are ever involved.

**Proposed fix**

- The conversion depends only on the schema string, so the result can be cached (keyed on that string).

**Measured impact**

- `scan().plan_files()` on an unpartitioned 150-manifest table: **~86 ms → ~48 ms (~1.8× faster)**.
- The saving grows as the number of manifests increases.

I am willing to contribute for this improvement.

从 pyiceberg/avro/file.py 中的 AvroFileHeader.get_schema() 开始，跟踪 avro_to_iceberg 转换。使用重复的 manifest 调用 scan().plan_files()，观察重复的工作，并确认转换结果会按 schema 字符串复用，同时随着 manifest 数量增加，规划性能得到提升。

由索引模型根据 Issue 内容生成。