feat(info): Fetch Spot Asset Context (vol, mid price, mark price, etc)

[saucepoint]

Added info.spot_asset_ctx("UBTC", "USDC") which returns a spot asset context:

{
    dayNtlVlm: float string,
    markPx: float string,
    midPx: Optional(float string),
    prevDayPx: float string,
    circulatingSupply: float string,
    coin: str
}

Under the hood, it calls /info, {type: "spotMetaAndAssetCtxs"} and does client-side filters & joins between spot metadata (tokens & markets) and context