Avro schema is re-converted to an Iceberg schema on every manifest read during scan planning #3662

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.