Glue schema conversion should not fail for display-only Iceberg types #3375

Problem

GlueSchemaBuilder converts Iceberg types into Glue column type strings. These strings are informational for Iceberg tables: readers use Iceberg table metadata as the source of truth.

The current Rust converter returns FeatureUnsupported for some Iceberg types such as Unknown, Timestamptz, and TimestamptzNs. That can prevent otherwise valid Iceberg table operations merely because Glue cannot represent the type as a native processing type.

The Java IcebergToGlueConverter explicitly documents this conversion as display-only and always produces a string, while retaining Glue/Hive-compatible spellings for known types:
https://github.com/apache/iceberg/blob/main/aws/src/main/java/org/apache/iceberg/aws/glue/IcebergToGlueConverter.java#L294-L350

Proposed behavior

Refactor the Glue conversion so every Iceberg type has a non-failing display representation, while preserving existing Glue-compatible mappings such as long to bigint, time/UUID to string, and fixed to binary.

Raised from review discussion on #2933:
#2933 (comment)