Function datetime_struct_fields 

pub fn datetime_struct_fields() -> Fields

Returns the canonical struct field definitions for DateTime encoding in Arrow.

DateTime is encoded as a 3-field struct to preserve timezone information:

• nanos_since_epoch: i64 nanoseconds since Unix epoch (UTC)
• offset_seconds: i32 seconds offset from UTC (e.g., +3600 for +01:00)
• timezone_name: Optional IANA timezone name (e.g., “America/New_York”)