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.

pyiceberg/avro/file.py の AvroFileHeader.get_schema() から始め、avro_to_iceberg 変換を追跡します。繰り返し同じマニフェストを使って scan().plan_files() を実行し、同じ処理が繰り返されることを観察して、変換結果がスキーマ文字列によって再利用されることを確認します。マニフェスト数が増えるにつれて、プランニングのパフォーマンスが向上します。

索引モデルが issue の本文から書いたものです。