add: deposit collateral - allow users to hold in the currency of the loan

the users' collateral for a margin trade is currently always held in the collateral currency
we need to allow collateral to be held in the currency of the loan
e.g. go rBTC long (trade on iSUSD loan token) but have your collateral held in sUSD (now it is converted to the collateral token - rBTC)
this leads to changing the leverage computation
now, it's clear that the collateral will be converted to rBTC, so if i go rBTC long 3x, i just need to borrow 2x the amount i'm providing, because my collateral is in rBTC (counting towards the position size)
and the collateral would not count towards the position size if the collateral held in the loan token currency - sUSD in the provided example

that's the way many exchanges have it, so some users are used to it
also they're liquidated less quickly
because their collateral doesn't decrease in value if the market is moving against them