Add missing type conversion methods to complement the existing to_int64()
method, providing complete coverage for all DuckDB data types:

- Integer types: to_int8, to_uint8, to_int16, to_uint16, to_int32, to_uint32, to_uint64
- Large integer types: to_hugeint, to_uhugeint, to_varint
- Numeric types: to_decimal, to_float, to_double
- Temporal types: to_date, to_time, to_time_tz, to_timestamp, to_timestamp_tz,
  to_timestamp_s, to_timestamp_ms, to_timestamp_ns, to_interval
- Other types: to_bool, to_varchar, to_blob, to_bit, to_uuid
- Complex types: to_list, to_map_entries, to_struct_properties
- Utility methods: value_type

All methods follow the same unsafe pattern as the original to_int64()
implementation, directly wrapping the corresponding duckdb_get_* C API calls.

This enhancement provides a complete and consistent API for extracting
values from DuckDB result sets in Rust applications.