Review margin 'source' base abstraction for buying power models

Expected Behavior

• Crypto/CryptoFuture/CashBuyingPowerModel use their collateral as source of margin/buying power, versus base BuyingPowerModel whos margin is the whole portfolio value. Believe there's room for a potential shared abstraction, 'pool' of margin. This would add support for the SetHoldings concept against the 'pool' and reduce some duplication

Actual Behavior

• There's some duplication happening see CryptoFutureMarginModel & I believe CashBuyingPowerModel too
• SetHoldings is calculated always against the total portfolio value/buying power

Potential Solution

• At https://github.com/QuantConnect/Lean/pull/6836 see https://github.com/QuantConnect/Lean/pull/6836/commits/ee479c127dbad39636900ad9b71f547bb139f4f1

Reproducing the Problem

N/A

System Information

N/A

Checklist

• I have completely filled out this template
• I have confirmed that this issue exists on the current master branch
• I have confirmed that this is not a duplicate issue by searching issues

• I have provided detailed steps to reproduce the issue

Start by comparing CryptoFutureMarginModel and CashBuyingPowerModel with the base BuyingPowerModel, focusing on collateral, margin pools, and SetHoldings calculations. Review PR #6836 and commit ee479c127dbad39636900ad9b71f547bb139f4f1. Done should mean the abstraction direction is agreed and the duplicated behavior is addressed.