feat(modify-leaverage): control position size

[0xosama]

Allow protocol to increase/decrease existing position leverage. as discussed with Drypto this is best done by controlling position size directly instead of specifying leverage.

Drypto:I think we need to change marginTrade to have it so people specify either amount borrowed or intended position size vs using the leverage slider. leverage slider is simpler but loses out integral position size handling that more sophisticated traders want/need
It isnt difficult to add but i have always preferred direct control on sizing as that is what I am used to using on venues such as IBKR, Binance, Bybit, etc. Sizing off of leverage is kind of backwards to me