Support Arrow Map schemas in Iceberg writes

[osipovartem]

Summary

• convert Arrow DataType::Map schemas to Iceberg MapType for write/create paths
• assign stable Iceberg field IDs recursively to map keys and values
• reject nullable Arrow map keys and malformed entries instead of producing invalid Iceberg metadata
• cover schema round-trip and missing-ID assignment, including Utf8View keys

Validation

• cargo +1.95.0 test -p iceberg-rust-spec arrow::schema --lib: 21 passed
• cargo +1.95.0 clippy -p iceberg-rust-spec --all-targets -- -D warnings
• cargo +1.95.0 fmt --all -- --check