[feature request] provide data type conversion between avro and iceberg data types

[kevinjqliu]

Feature Request / Improvement

see #2173 (comment) for more context

The thread address timestamp-millis conversion specifically from avro to iceberg. the complication is that iceberg spec does not support timestamp-millis natively, it supports timestamp-micros. Some conversion logic is needed

AvroSchemaConversion().avro_to_iceberg is currently only used to convert avro data types to those supported by iceberg spec

Also related to #2226

[matthias-Q]

This has to also be considered:

iceberg-python/pyiceberg/utils/schema_conversion.py

Line 168 in 4cac691

# For the Iceberg spec it is required to set the default value to null

In pure AVRO this strict requirement on the order of union types does not exist.