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.

Iniziate in pyiceberg/avro/file.py, in AvroFileHeader.get_schema(), e seguite la conversion avro_to_iceberg. Usate scan().plan_files() con manifest ripetuti per osservare il lavoro ripetuto e confermare che i risultati della conversione vengano riutilizzati in base alla stringa dello schema, con prestazioni di pianificazione migliori all'aumentare del numero di manifest.

Scritto dal modello di indicizzazione a partire dal testo della issue.