Cross-asset collateral

[davidsiska-vega]

With the addition of spot markets it would be nice if a party can hold e.g. wETH as collateral for ethUSDT settled market.

What this needs is, at the very least:

1. the per-market margin accounts becomes multiple accounts for multiple assets (presumably which ones are ok is configurable per market and relies on existing spot market to convert between the additional collateral assets and the settlement asset.
2. when comparing all the collateral balances to maintenance margin we probably use the spot mark price and a "liquidity charge" so 1 ethUSDT worth of wETH (in terms of mark price) would would only contribute say 0.8 of ethUSDT for the purpose of comparison with maintenance margin level.

All of this is TBD and milestone set.