Update the Loan data structure. Add `is_loan_withdrawn` for partial loan withdrawal.

[zeapherine]

The new Loan structure is as follows. add is_loan_withdrawn boolean field.

struct Loan {
id: felt,
owner: felt,
market: felt,
commitment: felt,
amount: Uint256,
current_market: felt,
current_amount: Uint256,
is_loan_withdrawn: felt,
debt_category: felt,
state: felt,
l3_integration: felt,
created_at: felt,
}