Function register_streaming_functions_with_watermark 

pub fn register_streaming_functions_with_watermark(
    ctx: &SessionContext,
    watermark_ms: Arc<AtomicI64>,
)

Registers streaming UDFs with a live watermark source — same as register_streaming_functions but watermark() reads watermark_ms (in milliseconds since epoch; values < 0 mean “no watermark”, returning NULL).