Function parse_ws_twap_slice_fill 

pub fn parse_ws_twap_slice_fill(
    item: &WsTwapSliceFillData,
    user: &str,
    is_snapshot: bool,
    instrument: Option<&InstrumentAny>,
    ts_init: UnixNanos,
) -> Result<HyperliquidTwapSliceFill>

Parses one userTwapSliceFills item into custom data.

Unknown coins leave instrument_id unset and do not fail the parse.