Support Iceberg decimals with 38-digit precision

[osipovartem]

Summary

• replace the 96-bit rust_decimal value used by Iceberg bounds with fastnum::D128
• support the full Iceberg decimal(38, s) value range in metadata and pruning statistics
• encode decimal bounds using the Iceberg minimum-length big-endian two's-complement representation
• keep DataFusion pruning and manifest statistics lossless through Arrow Decimal128

The approach adapts the decimal representation used by Apache iceberg-rust for full 38-digit precision to this fork's Value model.

Why

Snowflake can write 38-digit decimal lower/upper bounds into Iceberg metadata. Decoding those values through rust_decimal panicked because its mantissa is limited to 96 bits, so Rustice could not read otherwise valid Snowflake Iceberg tables.

Verification

• cargo +1.94.0 test -p iceberg-rust-spec --lib (151 passed, 1015 explicitly ignored)
• cargo +1.94.0 test -p datafusion_iceberg --lib (50 passed)
• cargo +stable clippy --all-targets --all-features -- -D warnings
• cargo +stable fmt --all -- --check
• regression coverage for positive and negative 38-digit byte round trips and conversion to DataFusion Decimal128